Effects of saphenous nerve stimulation and hemorrhage on discharge activity of parvocellular neurosecretory (PC) cells of the paraventricular hypothalamic nucleus (PVN) were studied in anesthetized rats. The PC cells were identified in the PVN with the criteria that units recorded in the PVN showed an antidromically conducted spike after median eminence but not after posterior pituitary stimulation of 0.45 mA intensity and were demonstrated histologically to be located in the parvocellular regions of the PVN. Saphenous nerve stimulation at 4 Hz repetition and of 3 mA intensity increased the discharge rate of 13 of the 52 PC cells tested. These excited cells exhibited a synaptically mediated excitation during post-stimulus period on peristimulus time histograms constructed during 1 Hz stimulation. Hemorrhage (4 ml/kg b.w.) activated 7 of the 10 PC cells tested which had been excited by saphenous nerve stimulation. Both saphenous nerve stimulation and hemorrhage increased immunoreactive adrenocorticotropic hormone (ACTH) in the plasma in the anesthetized rats. These data support the view that synergistic potentiation of ACTH secretion reported to occur after noxious and hypovolemic stimuli depends at least in part on the interaction between afferent neural signals originating from noxious and cardiovascular receptors on their way up to corticotropin-releasing factor (CRF)-secreting PVN neurons.