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To compare the blood-pressure-lowering effects of an angiotensin converting enzyme inhibitor, perindopril, with those of an angiotensin II type 1 receptor antagonist, L-158,809, for adult spontaneously hypertensive rats.A cross-over design was used, to treat adult spontaneously hypertensive rats with one drug for 10 weeks, and then with the other for 5 weeks.Adult, male spontaneously hypertensive rats (aged 15 weeks) were treated daily by gavage for 10 weeks with perindopril (P group) or L-158,809 (L group), then treatment was crossed over so that rats in the P group were treated with L-158,809 (P/L group) and rats in the L group were treated with perindopril (L/P group) for 5 weeks. Blood pressure was measured weekly. Plasma angiotensin converting enzyme activity, renal angiotensin receptor density, and arterial structure and functioning were measured after the single and crossover treatment periods.Treatment lowered the blood pressure from 206 ± 2 mmHg in rats in the control group, to 126 ± 2 in rats in the P group and 150 ± 2 in rats in the L group. After the cross-over period, blood pressure decreased further from 150 ± 2 to 129 ± 3 mmHg in rats in the L/P group, whereas blood pressure of spontaneously hypertensive rats in the P/L group increased from 126 ± 2 to 148 ± 2 mmHg. Perindopril treatment almost abolished plasma angiotensin converting enzyme activity, whereas L-158,809 treatment had no effect. Renal angiotensin II receptor density was decreased versus baseline in rats in the P and L groups. The affinity of binding was decreased versus baseline in rats in the L group. A positive correlation to blood pressure was found for mesenteric artery wall thickness and wall: lumen ratio. Concentration for half-maximal effect for the response of mesenteric arteries from rats in the P group to norepinephrine was lower than that of the control group rats. Angiotensin II potentiated the norepinephrine-stimulated contraction of arteries from rats in the control and P groups, but not that of arteries from rats in the groups treated with L-158,809.Perindopril was more effective than was L-158,809 at lowering the blood pressure of adult spontaneously hypertensive rats, and at altering the structure and functioning of the arteries.