Natriuretic and hypotensive effects of synthetic alpha-human atrial natriuretic polypeptide (alpha-hANP) were investigated in anesthetized spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY). The renal excretory and hypotensive effects of alpha-hANP were strengthened in a dose-dependent manner in both strains of rat. The natriuretic response to three doses of alpha-hANP (0.3, 1.0 and 3.0 micrograms/kg) were significantly greater in SHR than in WKY. These results suggest that atrial natriuretic polypeptide may play some role in water-electrolyte balance and control of blood pressure in SHR.