Infusion of bradykinin (1 microgram/min) or saline vehicle for 30 minutes into alloxan-diabetic rats produced no change in the very high levels of blood glucose. Furthermore, intravenous injection of captopril (3 mg/Kg body weight) into the diabetic rats did not result in a significant change of glucose concentrations over a period of 60 minutes. However, infusion of bradykinin 15 minutes after intravenous injection of captopril resulted in a marked decrease of glucose levels (p less than 0.01, compared to pretreatment) by 20 minutes after the start of the infusion. Thus, the captopril potentiated the effect of the kinin, possibly by inhibition of kininase II. Using a spectrophotometric assay with Bz-Gly-Gly-Gly as substrate insulin was shown to be an inhibitor of kininase II purified from hog lungs with an I50 of 1.6 X 10(-5)M compared to captopril with I50 of 2.2 X 10(-9)M. Furthermore, it was found that in vivo infusion of as little as 50 mU insulin over a period of 30 minutes, a dose that by itself was ineffective, potentiated the glucose-lowering activity of a bradykinin infusion in alloxanized rats. Interestingly, the infusion of insulin 15 minutes after injection of captopril, at doses of each compound which alone were inactive, did produce a significant fall (p less than 0.005) in glucose concentrations. Overall, the results show that captopril, insulin and bradykinin can interact to promote a reduction in blood glucose of alloxan-diabetic rats.