The effect of furosemide on plasma renin, vasopressin (AVP), and aldosterone concentrations was studied in 10 control and 6 nephrectomized lambs during the 1st 2 wk of life. In a separate study in 10 newborn lambs, 1-sarcosine-8-alanine-angiotensin II (saralasin acetate, 5 mug/kg per min) was infused alone for 40 min, after which furosemide 2 mg/kg i.v. was injected in association with continuing saralasin acetate infusion. Plasma renin activity increased from a mean (+/-SEM) of 21.3+/-3.4 ng/ml per h in the 10 control lambs to 39.4+/-8.2 ng/ml per h at 8 min (P < 0.001) and remained high through 120 min after furosemide. Plasma AVP and aldosterone concentrations increased from respective mean values of 2.1+/-0.4 muU/ml and 12.8+/-2.5 ng/dl to 9.8+/-2.0 muU/ml (P < 0.01) and 23.0+/-7.7 ng/dl (P < 0.05) at 35 min and 13.8+/-2.1 muU/ml and 23.0+/-4.4 ng/dl at 65 min after furosemide (each P < 0.01). There was an insignificant AVP response in the 10 lambs treated with angiotensin inhibitor: from a mean base line of 4.7+/-0.9 to 8.3+/-2.0 muU/ml at 35 min, and 7.4+/-2.0 muU/ml at 65 min after furosemide. There was no increase in AVP in the anephric lambs. The mean increment AVP response from base line in the newborn lambs without saralasin, Delta 10.8+/-2.0 muU/ml, was greater than in the lambs with saralasin, Delta4.0+/-1.9 (P < 0.05), and greater than in the anephric lambs, Delta3.3+/-2.1 muU/ml (P < 0.05). The mean blood pressure fell 6 mm Hg in the 10 control lambs (P < 0.05), 7 mm Hg in the anephric lambs (P < 0.05), and 16 mm Hg in the lambs treated with angiotensin inhibitor (P < 0.05) by 35 min after furosemide. However, the changes in plasma AVP were not related to the fall in blood pressure. These data support the view that the observed AVP response to furosemide in the newborn lamb was mediated through the renin-angiotensin system.