Extracellular multi-unit activity was recorded from sites in the supramammillary area of urethane anaesthetized rats, while simultaneously recording the hippocampal EEG. The pattern of supramammillary area cell discharge was found to be rhythmic and in phase with concurrent hippocampal rhythmical slow activity (RSA). Septal application of procaine (20%, 1-5 microliters), that abolished hippocampal RSA, did not abolish the rhythmicity of supramammillary cell firing. These data suggest that the frequency of hippocampal RSA may be determined in the supramammillary area rather than in the medial septum as has previously been supposed.