This study investigated the ability of the selective angiotensin II (AII)-type (AT)1 receptor antagonist losartan to reverse the fast (< 30 sec) pressor effect of AII, and the hypertension produced by chronic (2 weeks) i.v. infusion of AII (AII hypertension). We hypothesized the following: if AII hypertension is caused solely by stimulation of AT1 receptors mediating the fast pressor effect, then the time course of the antihypertensive effect of losartan in AII hypertension should parallel the time course of losartan inhibition of pressor responses to acute, bolus injection of AII. Thus, in one group of rats, pressor responses to bolus injections of AII (10 ng, n = 10) were measured before and subsequently at numerous time points after losartan administration (3 mg.kg-1 i.v.). Other groups of rats received continuous infusions of AII i.v. for 15 days at 2 ng.min-1 (n = 8), 4 ng.min-1 (n = 8) or 10 ng.min-1 (n = 6). On days 2, 7 and 12 of the AII infusion, rats received a bolus injection of losartan (3 mg.kg-1, i.a.). Mean arterial pressure (MAP) was then measured at numerous time points after losartan administration. Within 5 min of administration, losartan caused almost complete inhibition of fast pressor responses to acute injections of AII, and the magnitude of this inhibition did not change for over 24 hr. On the other hand, in AII hypertension, losartan lowered MAP significantly within 5 min only in rats receiving 10 ng.min-1 AII, but in all three groups caused a slower decline in MAP that peaked around 2 hr after losartan injection.(ABSTRACT TRUNCATED AT 250 WORDS)