Nonphasically firing activities were recorded from neurons in the supraoptic (s.o.) and paraventricular (p.v.) nucleus of the virgin female, male, and ovariectomized rat hypothalamic slice preparations. Following bath application of oxytocin (OXT), less than 1.0 x 10(-8) M, 24 (75%) of 32 virgin female s.o. neurons showed inhibitory responses and only one neuron showed excitatory response. In contrast to virgin female, five (63%) of eight male s.o. neurons were excited by OXT application. The same tendency was shown in the p.v. neurons. Among six s.o. neurons obtained from ovariectomized virgin rats, five neurons increased their firing rate to the application of OXT. After blocking synaptic transmission, the inhibitory responses of virgin s.o. neurons and the excitatory responses of ovariectomized virgin s.o. neurons to application of OXT were still observed with equal configurations. These findings suggest that the responses of nonphasic neurons to OXT may not due to synaptic drives.