Normotensive rats were chronically given 100 mg.kg-1.day-1 of captopril in their drinking water beginning 1 day before uninephrectomy. Mean blood pressure was significantly decreased 4 (17%) and 8 (18%) wk later in treated rats vs. age-matched controls. When histological techniques were used, a marked reduction of medial-intimal area of the abdominal aorta was found in treated rats at 4 (24%) and 8 (15%) wk without significant change in internal diameter. Cremaster arteriolar dimensions were measured by intravital microscopy after topical application of 10(-3) M adenosine. Structural diameter reductions occurred in large arterioles starting at 4 wk in treated rats and in small arterioles at 8 wk. Despite a significant increase in wall-to-lumen ratio of some large and small arterioles in treated rats, the wall cross-sectional area of these vessels was significantly decreased by captopril. Measured by stereological techniques, small arteriolar density decreased 17% at 4 wk and 13% at 8 wk in treated rats. Although decreased blood pressure may contribute to smaller aortic cross-sectional wall area, the effect of captopril was probably the major cause for smaller arteriolar lumens, reduced arteriolar cross-sectional wall areas, and arteriolar rarefaction.