Three distinct molecular weight species of active renin, with apparent molecular weights of more than 150,000, 65,000, and 43,000 are present in the plasma of conscious rats. After acute in vivo renin stimulation (ether anesthesia and hemorrhage), however, only the low molecular weight form could be detected. If, on the other hand, when diabutal was given and the duration of anesthesia was extended to 1 h to allow the active plasma renin level to return toward control, two species of renin were observed (greater than 150,000 and 43,000). Acid dialysis (pH 3.3) of normal plasma resulted in a significant increase in renin concentration, confirming the presence of an inactive renin (prorenin) in this species. Acid dialysis was shown to induce a decrease in the relative proportion of the high molecular weight form of the enzyme (greater than 150,000), with a concomitant increase in the 43,000 molecular weight species. Dialysis to pH 1.5 of plasma from 30-h nephrectomized rats to inactivate renin and destroy renin substrate resulted in the generation of an acid-stable factor which, when added to normal rat plasma, caused renin activation at pH 7.4. This renin- and renin substrate-free dialysate (pH 1.5) did not, however, alter the rate of angiotensin I generation when added to plasma samples devoid of inactive renin. Activation of renin in normal plasma could also be induced at pH 7.4 by the addition of plasma from nephrectomized rats which had previously been dialyzed to pH 3.3. These results indicate that the activation of renin by acid dialysis is not directly mediated by acid conditions and confirm the existence of an endogenous plasma factor which, after acid dialysis, is capable of converting inactive to active renin.