The product of the ob gene protein, leptin, has been suggested to function as an endogenous mediator of the cardiovascular system via sympathetic nerve activity. Moreover, extensive distribution of leptin receptor-like immunoreactivity has been demonstrated in the choroid plexus, cerebral cortex, hippocampus, thalamus and hypothalamus, especially in the paraventricular nucleus (PVN) and supraoptic nucleus (SON). In this study, we have investigated the in vivo effects of leptin on plasma arginine-vasopressin (AVP) secretion and the level of AVP messenger ribonucleotic acid (AVP mRNA) in the SON of conscious rats. Intracerebroventricularly administered leptin increased plasma AVP concentration in a dose-dependent manner (0–400 pmol/rat). The maximal effect was obtained at 15 min after the administration of leptin. Furthermore, in Northern blot analyses, the levels of AVP mRNa in the SON increased approximately 2-fold from the basal level after the administration of leptin. AVP mRNA expression in the PVN was also increased by leptin. However, leptin had no effects on plasma oxytocin (OXT) secretion and OXT gene expression in the SON. In conclusion, leptin is involved in AVP secretion via the central nervous system, however, its physiological role is unknown.