The antihypertensive effect of naftopidil (KT-611) following single oral administration was investigated in normotensive Wistar Kyoto rats (WKY), spontaneously hypertensive rats (SHR), DOCA-Salt hypertensive rats (DHR), 2-kidney 1-clip renal hypertensive rats (RHR) and Grollman type renal hypertensive dogs with 1-kidney (RHD); and it was compared with that of the selective alpha 1-adrenoceptor antagonist prazosin. The blood pressure and heart rate were measured under the unanesthetized, unrestrained state through an arterial catheter that was chronically implanted into the abdominal aorta. In SHR and WKY, both KT-611 (10 and 30 mg/kg, p.o.) and prazosin (1 and 3 mg/kg, p.o.) markedly inhibited the pressor response to the alpha 1-adrenoceptor agonist phenylephrine (3 micrograms/kg, i.v.). KT-611 (10 to 100 mg/kg, p.o.) showed a dose-dependent hypotensive effect in SHR, DHR and RHR but not in WKY. The hypotensive effect of KT-611 reached maximum at 0.5-1 hr, lasted for 4-6 hr and was more potent in DHR and RHR than in SHR. The potency of KT-611 was 1/10-1/30 weaker than that of prazosin. In RHD, single oral administration of KT-611 (1 to 10 mg/kg) caused a dose-dependent and long-lasting hypotensive effect. These results suggest that KT-611 has a long-lasting hypotensive effect in experimental hypertensive animal models.