Inhibitory interneurons often generate synchronous activity as an emergent property of their interconnections. To determine the role of electrical synapses in such activity, we constructed mice expressing histochemical reporters in place of the gap junction protein Cx36. Localization of the reporter with somatostatin and parvalbumin suggested that Cx36 was expressed largely by interneurons. Electrical synapses were common among cortical interneurons in controls but were nearly absent in knockouts. A metabotropic glutamate receptor agonist excited LTS interneurons, generating rhythmic inhibitory potentials in surrounding neurons of both wild-type and knockout animals. However, the synchrony of these rhythms was weaker and more spatially restricted in the knockout. We conclude that electrical synapses containing Cx36 are critical for the generation of widespread, synchronous inhibitory activity.