This study was performed in order to investigate the effect of diltiazem, which is a $Ca^{2+}$ channel blocker of benzothiazepine derivatives, on renal function in the dog. Diltiazem, when infused into the vein or carotid artery, produced the antidiuresis accompanied with the decreased excretion rates of sodium and potassium in urine$(E_{Na},;E_K)$ and the increased reabsorption rates of sodium and potassium in renal tubules$(R_{Na},;R_K)$. Diltiazem, when infused into a renal artery, exhibited the diuresis along with the increased renal plasma flow(RPF), osmolar clearance$(C_{osm})$, $E_{Na}$ and $E_K$, and decreased $R_{Na}$ and $R_K$ in only infused kidney. Above results suggest that diltiazem possess both antidiuretic action through central action and diuretic action by direct inhibition of electrolytes reabsorption rates in renal tubules, mainly distal tubule.