1. The effects of the ACE inhibitor, captopril, on collagen metabolism in spontaneously hypertensive rats (SHR) with cardiac hypertrophy was examined. Captopril (100 mg/kg per day) was administered in drinking water to 20 week old male SHR for 12 weeks. Collagen concentration was calculated from hydroxyproline content, and relative proportions of types I, III and V collagen were determined by non-interrupted SDS-polyacrylamide gel electrophoresis (SDS-PAGE). These parameters were examined in age and sex matched Wistar-Kyoto (WKY) rats, as well as in non-treated SHR, and compared with those of captopril-treated SHR. 2. Captopril significantly reduced both blood pressure (191 +/- 12.1 vs 146 +/- 11.2 mmHg, P < 0.01), and the ratio of left ventricular (LV) weight to bodyweight (BW; 2.38 +/- 0.17 vs 2.05 +/- 0.12 mg/g, P < 0.01). There were no significant differences in collagen concentration among WKY rats, captopril-treated SHR and non-treated 32 week old SHR. However, total collagen content in captopril-treated SHR reduced significantly compared with non-treated 32 week old SHR (16.8 +/- 2.0 vs 21.3 +/- 0.8 mg, P < 0.01). The relative proportion of type V collagen was significantly higher in both captopril-treated (58.6 +/- 3.4 vs 46.8 +/- 1.3%, P < 0.01) and non-treated 32 week old SHR (59.9 +/- 3.1 vs 46.8 +/- 1.3%, P < 0.01) compared with WKY rats. However, there were no significant differences between captopril-treated SHR and non-treated 32 week old SHR.(ABSTRACT TRUNCATED AT 250 WORDS)