The paraventricular nucleus (PVN) of hypothalamus is a major integrative center in homeostatic control.Morphological studies have revealed a high level of secretin and secretin receptor expression in the PVN.To investigate the direct electrophysiological effects of secretin in the PVN, in vivo extracellular recordings were performed in the present study.In 24 out of the 46 paraventricular neurons, micro-pressure ejection of secretin increased the firing rate from 3.07±0.43 Hz to 4.86±0.70 Hz.In another 8 out of the 46 paraventricular neurons,secretin decreased the firing rate from 2.61±0.46 Hz to 1.41±0.25 Hz.In the remaining 14 paraventricular neurons,secretin did not alter the firing rate significantly.The present findings provided direct electrophysiological evidence for the possible functions of secretin in the PVN.