AbstractWe have reported previously that angiotensin II increases urine flow and Na and K excretion rates in the aglomerular goosefishLophius americanus. The present experiments were undertaken to determine if antagonism of endogenous angiotensin has the opposite renal effects. Arterial blood pressure, urine flow, and plasma and urine electrolytes were measured during two consecutive clearance periods in two groups of goosefish. The control group (n = 7) received saline intravenously during both periods; the experimental group (n = 8) received saline during the first period and a competitive antagonist of angiotensin II (saralasin, or [Sar1, Val5, Ala8] angiotension II) dissolved in saline during the second period. The control group exhibited no significant change over time in any of the measured variables: arterial blood pressure; plasma [Na], [K], [Mg], [Ca]; osmolality; urine flow; and excretion rates of Na, K, Ca, and Mg. Arterial blood pressure and plasma electrolytes and osmolality were stable over time in the experimental group also. However, during saralasin infusion there were significant decreases in urine flow and in excretion rates of K, Mg, and Ca. Taken together with our previous results, these observations suggest that endogenous levels of angiotensin in goosefish are sufficient to have direct renal effects. The mechanisms of action of angiotensin are not clear, but involve neither changes in systemic blood pressure nor filtration but might involve inhibitory effects of angiotensin on tubular reabsorptive processes.