This study was performed to elucidate the mechanism of antidiuretic action of diltiazem by infusion into the vein and carotid artery, of diuretic action into a renal artery in dog. Renal denervation caused a reversal of the effect of diltiazem from the antidiuretic to the diuretic when infused into vein or carotid artery, and potentiated the diuretic effect when infused into a renal artery. The changes of renal function in diuretic circumstances as described above included the increase in renal plasma flow, osmolar clearance, the amounts of sodium and potassium excreted in urine and the decrease in reabosrption rate of sodium and potassium in renal tubules. Above results suggest that antidiuretic action of diltiazem may be mediated by central nervous system, not by endogenous substance, diuretic action by direct renal action.