We have studied the effects of bilateral nephrectomy and adrenalectomy on angiotensinogen concentration, plasma renin activity, and total plasma renin activity obtained after trypsin activation in rats and compared with controls. Our results show that the plasma angiotensinogen concentration of nephrectomized (NX) rats is 5-fold higher than that of adrenalectomized (AX) rats. On the other hand, plasma angiotensinogen concentration of AX rats was about 2.5-fold lower than that of control rats. While NX rat plasma possess no renin activity, its mixing (1:1, v/v) with AX plasma results in 2-fold increase in renin activity over that observed for AX plasma alone. These results suggest that the apparent increase in renin activity upon mixing these two plasmas is at least partly due to an overall increase in angiotensinogen concentration in the mixture. To show that it is not due to activation of NX plasma prorenin by a convertase from AX plasma, prorenin-free NX rat plasma was prepared by using an anti-renin immunoaffinity column. When this prorenin-free NX plasma was mixed (1:1, v/v) with AX, again a 2-fold increase in renin activity was observed which is attributed to the overall increase in plasma angiotensinogen in the mixture. It is concluded that rat plasma prorenin is probably not activated within the circulation by a prorenin convertase from the rat kidney.