To explore the effect of chronic converting enzyme inhibition on the macro- and microcirculation, normotensive rats were chronically given 100 mg/kg/day of captopril in their drinking water beginning one day before uninephrectomy. Cremaster arteriolar dimensions were measured 2, 4, or 8 weeks later by intravital microscopy, before and after topical application of 10(-3) M adenosine. Mean blood pressure were significantly decreased at 4 (17%) and 8 (18%) weeks in treated rats vs age-matched control. Structural diameter reductions occurred in large arterioles starting 4 weeks in treated rats, and in small arterioles at 8 weeks. The cross-sectional wall area of large arterioles increased with age in control animal, but not in captopril treated one. Eight weeks of captopril treatment also decreased the cross-sectional wall area in small arterioles. Measured by stereological techniques, small arteriolar density decreased 17% at 4 weeks and 13% at 8 weeks in treated rats. Using histological techniques, a marked reduction of medial-intimal area of the abdominal aorta was found in treated rats at 4 (24%) and 8 (15%) weeks without a significant change in internal diameter.