Environmental enrichment improves hippocampus-dependent spatial learning and memory. However, how enrichment influences hippocampal network activity remains still largely unclear.Using cFos labeling, we showed that exploration of a novel context leads to increased cFos activity in hippocampal principal neurons in mice housed under both standard and enrichment conditions. Remarkably, the number of cFos-expressing cells was lower in enriched mice. Additionally, we recorded Ca2+ responses in CA1 pyramidal cells (PCs) using miniature microscope in mice performing exploration. We observed lower event frequency in PCs of enriched mice. It indicates that enrichment does not increase principal cell activity but promotes sparse coding in hippocampus.Next, we asked how enrichment affects GABAergic inhibition. We found that enrichment leads to increased activation of dendrite-targeting somatostatin-expressing (SOM) interneurons during exploration due to larger number of glutamatergic synapses onto SOM interneurons. It is accompanied by increased synaptic transmission between PCs and SOM interneurons, as shown by whole-cell patch-clamp recordings in SOM interneurons. Furthermore, we observed increased lateral feedback inhibition in enrichment.To understand how it translates in vivo, we performed simultaneous Ca2+ imaging from CA1 PCs and optogenetic silencing of SOM interneurons in mice during exploration. We found that SOM interneurons have a big impact on the event frequency of PCs and the number of active cells. Most importantly, the effect of silencing on the number of active cells was stronger in enrichment.Taken together, our data show that environmental enrichment decreases the size of hippocampal cell assemblies via enhanced recruitment of dendrite-targeting SOM interneurons.