diff --git a/src/content/app/regulatory-activity-viewer/RegulatoryActivityViewer.tsx b/src/content/app/regulatory-activity-viewer/RegulatoryActivityViewer.tsx
index d6d5af5dd3..b0bf576590 100644
--- a/src/content/app/regulatory-activity-viewer/RegulatoryActivityViewer.tsx
+++ b/src/content/app/regulatory-activity-viewer/RegulatoryActivityViewer.tsx
@@ -21,6 +21,8 @@ import { useAppSelector } from 'src/store';
 import { getActiveGenomeId } from 'src/content/app/regulatory-activity-viewer/state/general/generalSelectors';
 import { getMainContentBottomView } from 'src/content/app/regulatory-activity-viewer/state/ui/uiSelectors';
 
+import usePreselectedEpigenomes from './hooks/usePreselectedEpigenomes';
+
 import { StandardAppLayout } from 'src/shared/components/layout';
 import RegionOverview from './components/region-overview/RegionOverview';
 import RegionActivitySection from './components/region-activity-section/RegionActivitySection';
@@ -32,6 +34,7 @@ import SelectedEpigenomes from './components/selected-epigenomes/SelectedEpigeno
 
 const ActivityViewer = () => {
   const activeGenomeId = useAppSelector(getActiveGenomeId);
+  usePreselectedEpigenomes();
 
   return (
     <StandardAppLayout
@@ -56,6 +59,7 @@ const MainContent = ({ genomeId }: { genomeId: string | null }) => {
     <div>
       Hello activity viewer
       <RegionOverview activeGenomeId={genomeId} />
+      {/* The spacer divs below are temporary */}
       <div style={{ margin: '0.6rem 0' }} />
       <MainContentBottomViewControls genomeId={genomeId} />
       <div style={{ margin: '0.6rem 0' }} />
diff --git a/src/content/app/regulatory-activity-viewer/hooks/usePreselectedEpigenomes.ts b/src/content/app/regulatory-activity-viewer/hooks/usePreselectedEpigenomes.ts
new file mode 100644
index 0000000000..a279d564d5
--- /dev/null
+++ b/src/content/app/regulatory-activity-viewer/hooks/usePreselectedEpigenomes.ts
@@ -0,0 +1,93 @@
+/**
+ * See the NOTICE file distributed with this work for additional information
+ * regarding copyright ownership.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import { useEffect } from 'react';
+
+import { useAppDispatch, useAppSelector } from 'src/store';
+
+import { getEpigenomeSelectionCriteria } from 'src/content/app/regulatory-activity-viewer/state/epigenome-selection/epigenomeSelectionSelectors';
+import { getActiveGenomeId } from 'src/content/app/regulatory-activity-viewer/state/general/generalSelectors';
+
+import { useEpigenomeMetadataDimensionsQuery } from 'src/content/app/regulatory-activity-viewer/state/api/activityViewerApiSlice';
+import { addSelectionCriterion } from 'src/content/app/regulatory-activity-viewer/state/epigenome-selection/epigenomeSelectionSlice';
+
+import type { EpigenomeMetadataDimensions } from 'src/content/app/regulatory-activity-viewer/types/epigenomeMetadataDimensions';
+
+/**
+ * Upon the first load of epigenome metadata dimensions,
+ * check if user has selected any dimensions,
+ * and if not, then pre-select epigenomes using the default dimensions (if available).
+ *
+ * See the CommonMetadataDimensionFields type for epigenome metadata dimensions.
+ *
+ * NOTE: this hook should be only run once, at the top level of RegulatoryActivityViewer
+ */
+const usePreselectedEpigenomes = () => {
+  const activeGenomeId = useAppSelector(getActiveGenomeId) ?? '';
+  const { currentData: epigenomeMetadataDimensionsResponse } =
+    useEpigenomeMetadataDimensionsQuery();
+  const epigenomeSelectionCriteria = useAppSelector((state) =>
+    getEpigenomeSelectionCriteria(state, activeGenomeId)
+  );
+  const dispatch = useAppDispatch();
+
+  useEffect(() => {
+    if (!activeGenomeId || !epigenomeMetadataDimensionsResponse) {
+      // can't do anything until epigenomeMetadataDimensionsResponse is available
+      return;
+    }
+    if (Object.keys(epigenomeSelectionCriteria).length) {
+      // some epigenomes have already been selected; bail out
+      return;
+    }
+
+    const initialEpigenomeFilters = getPreselectedDimensions(
+      epigenomeMetadataDimensionsResponse.dimensions
+    );
+
+    if (initialEpigenomeFilters.length) {
+      for (const { dimensionName, value } of initialEpigenomeFilters) {
+        dispatch(
+          addSelectionCriterion({
+            genomeId: activeGenomeId,
+            dimensionName,
+            value
+          })
+        );
+      }
+    }
+  }, [
+    epigenomeMetadataDimensionsResponse,
+    epigenomeSelectionCriteria,
+    activeGenomeId
+  ]);
+};
+
+const getPreselectedDimensions = (dimensions: EpigenomeMetadataDimensions) => {
+  const result: { dimensionName: string; value: string }[] = [];
+
+  for (const [dimensionName, dimensionData] of Object.entries(dimensions)) {
+    if (dimensionData.default_values.length) {
+      for (const value of dimensionData.default_values) {
+        result.push({ dimensionName, value });
+      }
+    }
+  }
+
+  return result;
+};
+
+export default usePreselectedEpigenomes;
diff --git a/src/content/app/regulatory-activity-viewer/types/epigenomeMetadataDimensions.ts b/src/content/app/regulatory-activity-viewer/types/epigenomeMetadataDimensions.ts
index bf404072c4..d490770f3a 100644
--- a/src/content/app/regulatory-activity-viewer/types/epigenomeMetadataDimensions.ts
+++ b/src/content/app/regulatory-activity-viewer/types/epigenomeMetadataDimensions.ts
@@ -19,12 +19,12 @@ type CommonMetadataDimensionFields = {
   collapsible: boolean;
   filterable: boolean;
   zero_counts_visible: boolean;
+  default_values: string[];
 };
 
 type DimensionWithCategoricalData = CommonMetadataDimensionFields & {
   type: 'categorical';
   values: string[]; // <-- this is what makes the dimension "categorical"
-  default_values: string[];
 };
 
 type DimensionWithCategoricalDataAndOntology = CommonMetadataDimensionFields & {
@@ -36,7 +36,6 @@ type DimensionWithCategoricalDataAndOntology = CommonMetadataDimensionFields & {
       url: string;
     };
   }[];
-  default_values: string[];
 };
 
 // the only member of this type currently is age, which is still very uncertain
@@ -47,7 +46,6 @@ type DimensionWithCategoricalDataAndDescription =
       name: string;
       description: string;
     }[];
-    default_values: string[];
   };
 
 type EpigenomeMetadataDimension =