1. In plasma samples from normal subjects and patients with untreated essential hypertension, the concentration of inactive renin (as measured after acidification) was on average 4-5 times higher than the concentration of active renin (as measured without acidification).2. Plasma angiotensin II concentration was correlated to active renin but not to inactive renin. 3. A hyperacute stimulation induced by infusion of saralasin resulted in a marked rise of active renin, whereas inactive renin remained unchanged. 4. An acute stimulation induced by frusemide and ambulation led to a considerable rise in active renin and a slight, but significant, rise of inactive renin. 5. Stimulation with oral thiazide over 5 days induced a seven-fold rise of active renin, with a doubling of inactive renin. Thiazide treatment for 3 months led to a four-fold rise of active renin and a three-fold rise of inactive renin. 6. There was no difference between the concentrations of inactive renin in systemic plasma, ipsilateral and contralateral renal venous plasma in 12 patients with renovascular hypertension, neither before nor after infusion of saralasin with the associated fall in blood pressure. 7. We conclude that the time constants pertinent to secretion or release of active and inactive renin in man are of different orders of magnitude.