Endothelin (ET) is thought to be involved in the central regulation of body water metabolism. A recent perfusion study of rat hypothalamus showed that ET has a direct stimulatory action on arginine-vasopressin (AVP) release. This study was undertaken to investigate whether ET acts directly on AVP release for supraoptic nucleus (SON) neurons, using brain slices containing only the SON. It was demonstrated that ET inhibited the AVP release dose-dependently from 10(-11) M to 10(-6) M. No difference of the effects of ET-1 and ET-3 on AVP release was observed. These results suggest that ET directly inhibits SON neurons through the ETB receptor.