The synaptic interactions between catecholaminergic terminals and luteinizing hormone releasing hormone (LHRH)-containing neurons in the medial preoptic area of the rat was studied by electron microscopy using LHRH immunocytochemistry combined with 5-hydroxydopamine labeling or autoradiography after injection of 3H-dopamine or 3H-noradrenaline in the same tissue section. Axon terminals labeled with 5-hydroxydopamine, 3H-dopamine or 3H-noradrenaline were found to make synapse-like contacts with LHRH-immunoreactive nerve cell bodies and fibers in the medial preoptic area and also 5-hydroxydopamine-labeled terminals made synaptic contacts with the same LHRH-immunoreactive nerve fibers with unlabeled terminals. These findings suggest that catecholaminergic neurons may innervate LHRH-containing neurons to regulate LHRH secretion via synapses with other unknown neurons in the medial preoptic area of the rat.