Recent evidence suggests that the supramammillary nucleus of the posterior hypothalamus serves as an important relay in a brainstem to septum/hippocampus pathway involved in the generation of hippocampal theta rhythm. In order to examine the role of the supramammillary nucleus as a possible relay/mediator of hippocampal theta rhythm, electrolytic lesions and procaine injections were administered to the supramammillary nucleus of freely moving and urethane-anesthetized rats, respectively. In the urethane-anesthetized rat, it was found that procaine injections attenuated both the frequency and amplitude of theta rhythm elicited by stimulation of the pontine reticular formation. These data suggest that the pontine reticular elicitation of hippocampal theta rhythm is mediated through connections with the supramammillary nucleus. However, it was found that lesions of the supramammillary nucleus failed to produce significant changes in the hippocampal electroencephalogram of freely moving animals. Several explanations concerning this apparent discrepancy are discussed. The most compelling is that multiple brainstem to septum/hippocampus pathways may serve to generate or facilitate the generation of theta rhythm in the freely moving animal. The present report demonstrates that the supramammillary nucleus plays a questionable role in the mediation of hippocampal electroencephalogram signals which are thought to be important for mnemonic processes.