Plasma renin activity, aortic tissue renin, and plasma renin substrate concentrations were measured simultaneously and sequentially in groups of rats after (1) bilateral nephrectomy, (2) water deprivation, and (3) bilateral adrenalectomy. Following nephrectomy, plasma renin activity and aortic tissue renin fell in parallel fashion to undetectable or very low levels after 18 h, whereas the substrate concentration in plasma increased markedly. Rats deprived of water for 3 days, but receiving Purina Chow, showed a significant increase in plasma renin activity and in aortic tissue renin and a marked decrease in plasma substrate concentration. Bilateral adrenalectomy resulted in a. very marked augmentation of plasma renin activity and of aortic tissue renin with a concomitant suppression of plasma substrate concentration. These findings suggest that arterial tissue renin reacts to the same stimuli which modify renin activity in plasma.