The present study evaluated the role of Na+-K+ -ATPase stimulation in the relaxant response to synthetic atrial natriuretic factor (atriopeptin II; ANF) in norepinephrine (0.18 microM)-contracted rabbit aorta. ANF (10(-10)-10(-7) M) produced a concentration-related relaxation of NE-contracted aortic rings. The calculated concentration required to produce 50 per cent relaxation (EC50) was 5 +/- 3 X 10(-9) M. ANF-induced relaxation was significantly attenuated by ouabain (30 microM). Likewise, ANF (10(-7) and 5 X 10(-9) M) elicited significantly less relaxation of aortic rings contracted with norepinephrine in a physiological salt solution containing zero KCl. Nitroprusside-induced relaxation also was attenuated by ouabain and in zero KCl salt solution. The data suggest that synthetic ANF induces relaxation in rabbit aortic rings by a ouabain and KCl sensitive mechanism, presumably Na+-K+ -ATPase.