1. Intrarenal infusion of frusemide markedly stimulated renin secretion in the isolated perfused kidney of the rat. 2. Renin vales increased from 24+/-6 to 195+/-34 units of secretion rate (renin concentration (nmol angiotensin I/h per litre) X flow rate (ml/min)) following administration of frusemide for 8 min, compared with corresponding control values of 13 +/- 2 (P greater than 0.05) and 47 +/-18 (P less than 0.001). This stimulatory effect was also observed when urine flow was interrupted by ligation of the ureters. 3. The changes in renal perfusion pressure and perfusate flow rate were not significantly differnt from the control values. 4. These findings indicate the existence of an intrarenal site of action for frusemide on renin secretion. 5. Since frusemide did dot appear to alter perfusion pressure or flow rate, and was effective when urine flow was abolished, a dominant role for a vascular or macula densa receptor mechnaism seems unlikely. A direct effect of frusemide on the renin secreting cell is therefore suggested.