1. The effect of diazoxide (17-3 micronmol min-1 g-1) and frusemide (0-12 micronmol min-1 g-1) on renin secretion was examined in the isolated perfused rat kidney. These substances are potential renal vasodilators with opposite effects on urine sodium excretion. 2. Both agents significantly increased renin secretion rate above control values. In the case of frusemide this was not altered by ureteric occlusion and presumed absence of urine flow. 3. Mean renal perfusion pressure decreased to the same extent with diazoxide and frusemide infusion as in the control experiments and no additional vasodilatory effect was observed on the basis of changes in flow rate of perfusate. 4. These observations identify an intrarenal site of action for diazoxide and frusemide on renin secretion. The apparent independence of this stimulatory action on renal vasodilation and urine flow suggests a direct effect on the renin-producing cell.