In rats under urethane anaesthesia, brief tetanic stimulation of the medial septum produces a dual effect upon the field potentials elicited in the ammon's horn by commissural stimulation: a facilitation of the population spike recorded in the pyramidal layer and a depression of the field EPSP recorded in the apical dendrites. Both effects, which are also produced by local application of muscarinic agents, are antagonized by local administration of muscarinic antagonists.