Summary The effect of the recently obtained pure submaxillary gland renin on blood pressure and its action on rat plasma renin substrate to generate angiotensin were studied. Administration of subnanogram quantities of this renin to nephrectomized rats caused a sustained rise of blood pressure and decrease in sensitivity of the animal to angiotensin. The optimum pH of its reaction with substrate was between 8 and 8.5 and under appropriate conditions the rate of angiotensin generation was proportional to the amount of enzyme, substrate concentration, and time of incubation. The enzyme liberated angiotensin from plasma renin substrate of a number of species.