The effects of enalapril and sodium depletion on renin synthesis and secretion were studied in mice with a left hydronephrotic kidney caused by unilateral ureteral ligation (UUL). In the control animals, there was no difference in plasma renin concentration between the right and left renal veins. In mice with left ureteral ligation, the renin concentration in the vein draining the hydronephrotic kidney was similar to or lower than that in the aorta under control conditions and after either stimulation with enalapril or depletion of sodium. Enalapril and sodium restriction increased plasma renin concentration, and this increase was due to secretion from the nonhydronephrotic kidney. The renin concentration per gram of kidney tissue and the mRNA for renin per gram of kidney tissue were similar in both the control and hydronephrotic kidney, and the values rose 3-4-fold in both kidneys after enalapril or sodium depletion. Immunostaining for renin confirmed these findings and indicated that renin per glomerulus was higher in the hydronephrotic kidney. Thus, removal or reduction of angiotensin II activity or depletion of sodium stimulated synthetic activity to a similar extent in the normal and hydronephrotic kidneys; however, secretion from the kidney without a macula densa (hydronephrotic) was not increased. Thus, the signals that control synthesis and secretion are different, and for these stimuli, secretion appears to require an intact macula densa.