The effects of cortisol on neurons in the parvocellular division of the paraventricular nucleus (PVN) were investigated in hypothalamic slices prepared from adrenalectomized (ADX) and intact rats. In ADX rats, cortisol inhibited 14 (21%) out of 68 PVN neurons tested at a concentration of 10(-5) M and 11 (16%) of 69 PVN neurons tested at a concentration of 10(-4) M. In intact rats, cortisol at concentrations of 10(-5) M and 10(-4) M did not change the activity of most PVN neurons tested. We conclude that cortisol has an inhibitory effect on neurons in the parvocellular division of the PVN in ADX rats.