Neurons in the medial septal nucleus were recorded extracellularly in response to auditory stimuli in chloral-hydrate-anesthetized rats. Two populations of neurons were identified, both of which were localized to the nucleus by horseradish peroxidase labeling. The auditory-responsive population was characterized by slow axonal conduction velocity, as measured after antidromic activation from the fornix. This population probably represents cholinergic neurons with unmyelinated or thinly myelinated axonal projections to the hippocampus. The other population was not auditory-responsive and had relatively fast conduction times. This population was most likely GABAergic neurons, which have heavily myelinated axons. The timing of discharge within the medial septal nucleus suggests that its cholinergic neurons may regulate the response of the hippocampus to auditory stimuli by influencing the activity of both pyramidal cells and interneurons. The medial septal nucleus may thus play a critical role in the gating of the response to repeated auditory response in the hippocampus.