Rats with electrodes chronically implanted in the medial septal area and the hippocampus were trained to run in an alley for water reward. Low-frequency stimulation of the septal area was able to drive the hippocampal theta rhythm. Applied during extinction such theta-driving, at 7.7 Hz, decreased resistance to extinction, but applied during acquisition it increased subsequent resistance to extinction. During extinction, the higher the frequency of theta which an animal displayed, the more rapidly it extinguished; conversely, the higher the theta-frequency at the end of acquisition, the greater was the subsequent resistance to extinction. Theta-frequency was higher on nonrewarded trials than on rewarded trials and it was increased by extinction interpolated between phases of continuous reinforcement. It is proposed that this change in theta-frequency is the physiological basis of the partial reinforcement extinction effect, and that a theta rhythm indicates a functionally active hippocampus.