The sulfated and unsulfated cholecystokinin (CCK) octapeptide sequences and the pancreatic CCK antagonists, CR 1409 and benzotript, were administered iontophoretically while dentate gyrus granule cell activity was recorded in the anesthetized rat. During application of the compounds, the peri-stimulus time histogram (PSTH) was constructed of granule cell activity coupled to stimulation of the sciatic nerve. CCK and CR 1409, but not benzotript, were found to change significantly the PSTH by enhancing and prolonging the response to sensory stimulation. These results are interpreted as indicating that CCK can modulate impulse flow through the dentate gyrus.