SummarySix newborn lambs were studied during continuous infusion of saralasin acetate, 5 μg/kg/min for 135 min; 40 min after beginning saralasin, furosemide (2 mg/kg) was injected over 1–2 min. In addition, six anephric lambs were studied after injection of furosemide. Plasma renin activity (PRA) increased from 23 ± 2.7 ng/ml/hr (M and SE) to 85.8 ± 16.5 (P< 0.05) during infusion of saralasin alone and remained at this level after injection of furosemide. PRA did not increase above base line after injection of furosemide in the anephric lambs. Blood pressure dropped after saralasin infusion in the normal lambs (P< 0.05), and after furosemide injection (P< 0.05) in both groups. Plasma aldosterone concentrations did not increase in response to furosemide in either group. The results suggest that angiotensin II is important in maintaining blood pressure in the newborn and exerts antagonistic effects on the renal renin secretion mechanism.SpeculationThe renin-angiotensin-aldosterone system is activated in the normal newborn. Possible reasons for the activation include decreased circumferential tension of small arteries and salt wasting.