The persistent effects on blood pressure of the angiotensin II receptor antagonist losartan and the converting enzyme inhibitor captopril were compared in the young spontaneously hypertensive rat (SHR). Losartan (DuP753/MK954, 15 mg/kg/day) and captopril (100 mg/kg/day) were given in the drinking water of 3-week-old SHRs for 4- and 10-week durations. Blood pressure was measured during treatment and after treatment was stopped until the age of 30 weeks. Both losartan and captopril given for 4 and 10 weeks prevented the development of hypertension during treatment and redevelopment of hypertension after treatment was stopped. Treatment for 10 weeks was more effective than for 4 weeks in lowering long-term pressure. Four weeks of treatment did not affect the mesenteric resistance artery media/lumen (m₁/l₁) ratio. In contrast, both losartan and captopril given for 10 weeks resulted in large and significant reductions in m₁/l₁ [5.3 ± 0.8 and 5.63 ± 0.8 vs 7.7 ± 0.8 × 10^-2 (SD), p < 0.001]. In losartan-treated rats, plasma renin and angiotensin II concentration were increased between 4- and 7-fold at the end of both treatment periods. These findings show losartan to be an effective antihypertensive agent and support data implicating angiotensin II in the early events leading to hypertension in this model. The abilities of losartan and captopril to affect blood pressure without affecting vascular structure suggest that the latter is a poor predictor of long-term hypertensive levels in the SHR.