Intrarenal infusion of atrial natriuretic factor has been reported to increase renal blood flow and sodium excretion, and either to depress or not to affect renin release. We examined the effect of a synthetic atrial peptide, AP II, on renin release, renal perfusate flow and glomerular filtration rate in isolated rat kidneys, perfused at constant pressure with a synthetic Krebs-Henseleit hydroxyethyl-starch-albumin medium. Atrial peptide II, 0.3 to 30 nmol/l, produced a concentration-dependent increase in renin release. Renal perfusate flow increased initially but returned to pre-infusion levels during infusion. During vasodilatation by continuous infusion of hydralazine, AP II produced only minor changes of perfusate flow but higher increases of renin release than in controls, indicating that this effect is not secondary to vasodilatation. During suppression of prostaglandin synthesis by indomethacin, the effects of AP II were unaltered. Furthermore, AP II in concentrations up to 200 nmol/l did not alter resting membrane potential in juxtaglomerular cells.