It has been known that central tryptaminergic system is closely related with the regulation of renal function, and that central 5-HT1 receptors mediate diuresis and natriuresis, whereas central 5-HT2 and 5-HT3 receptors mediate antidiuresis and antinatriuresis. Among many subtypes of 5-HT1 receptors, central 5-HT1A subtype has been suggested to exert diuretic and natriuretic effets. Further, it was recently observed that TFMPP, 5-HT1B agonist, elicited profound diuresis and natriuresis when administered intracerebroventricularly(icv). Present study is therefore undertaken to delineate the mechanism involved in the natriuresis and diuresis induced by icv TFMPP, employing the denervated and vagotomized rabbits. The influence of icv TFMPP on the plasma level of ANP was also observed. TFMPP 250 microgram/kg icv produced marked diuresis and natriuresis. Renal hemodynamics showed significant increase only in the first 10-min period after administration and thereafter tended to recover. However, natriuretic action lasted even after the increased renal hemodynamics returned to the control level, suggesting the decreased Na reabsorption in the tubules by humoral natriuretic factors. Systemic blood pressure transiently increased. In rabbits in which one kidney is denervated, with the contralateral intact as the control kidney, the denervated kidney also responded with natriuresis and diuresis like that of the normal rabbit. The contralateral kidney responded with typical diuretic and natriuretic effects, along with the marked increased of renal hemodynamics. The plasma ANP, one of humoral natriuretic factors, increased after administration of icv TFMPP, peaking at about 15min. In bilaterally vagotomized rabbits, the natriuretic and diuretic effects produced by icv TFMPP were greater than that of the normal rabbits. These observations suggest that the natriuresis and diuresis elicited by icv TFMPP result from the inhibition of tubular Na reabsorption mainly through mediation of ANP. It has been also suggested that vagus nerve might exert inhibitory influence on the diuretic action of icv TFMPP, because the renal effects was augmented in the vagotomized rabbits.