Rhythmic multiunit activity, leading the negative peak of concurrently recorded dentate rhythmical slow-wave activity (RSA, theta) by a constant degree over a range of RSA frequencies, was observed in the supramammillary nucleus (SuM) after carbachol infusion (5%, 0.25-0.5 microliter) into the nucleus pontis oralis (RPO). As the rhythmic SuM activity survived bilateral transection anterior to SuM, its occurrence is not dependent on descending influences from the septum or hippocampus. Infusion of additional carbachol into the RPO resulted in epileptiform "sharp-waves" in the hippocampal EEG, and correlated nonrhythmic cell discharge in the SuM. Bilateral transection rostral to SuM abolished the sharp-wave related SuM activity (and restored SuM rhythmical activity). Thus, in contrast to RSA related rhythmical SuM discharge, sharp-wave related SuM discharge seems dependent on descending projections from the hippocampus.