Plasma renin reactivity (PRR) is the in vitro rate of angiotensin generation after addition of exogenous renin to plasma. The purpose of the present study is to compare measurements of PRR in venous effluent from the involved and uninvolved kidneys in both experimental and clinical renovascular hypertension. A two-kidney model of experimental hypertension was created by placing an ameroid resin constrictor around one renal artery in each of seven dogs. Plasma renin activity (PRA) in venous plasma from the involved kidney increased (p < 0.001); comparing PRA in venous effluent from the stenotic and nonstenotic kidneys, the PRA ratio also increased (p < 0.005). Renal venous PRR did not change on either side after occlusion of the renal artery (p > 0.1), and the renal venous PRR ratio did not differ from the mean control ratio of 1.0 ± 0.1 SE (p > 0.1). Similarly, in 9 patients with renovascular hypertension, mean PRR in venous plasma from the two kidneys did not differ (p > 0.8). These results suggest that measurement of renal venous PRR is not helpful in confirming a diagnosis of renovascular hypertension.