Semipurified soybean trypsin inhibitor added to rat and human plasma leads to a concentration dependent decrease in the rate of angiotensin I generation. This inhibition is due to binding of renin substrate to the inhibitor. Renin substrate present in nephrectomized rat plasma was more susceptible to binding than substrate of the normal rat suggesting structural differences in the substrate generated following nephrectomy. Because trypsin inhibition is necessary for measurement of active and inactive renin, we examined several alternate trypsin inhibitors. The Bowman-Birk inhibitor from soybean had similar actions as purified soybean trypsin inhibitor while trypsin inhibitors from lima bean and chicken did not depress renin substrate, but did have variable effects on the measured levels of active and total plasma renin. Surprisingly, crude soybean trypsin inhibitor did not suppress renin substrate and actually increased angiotensin I generation during PRA and PRC measurements. Since the crude preparation did not suppress renin substrate, changes in the specificity of the inhibitor may occur during its purification. The augmentation of PRA and PRC may be related to angiotensinase inhibitory actions.