OBJECTIVE: To determine the effect of angiotensin converting enzyme inhibition (CEI) upon renal function and the incidence of glomerulosclerosis in spontaneously hypertensive Fawn Hooded rats (FHR). DESIGN: Male FHR were treated with captopril from the age of 5 months when mild hypertension, proteinuria and glomerulosclerosis are present, and sacrificed at 12 months of age. Renal function was determined in separate groups of FHR at 6 months of age. METHODS: Proteinuria, body weight and systolic blood pressure were determined at regular intervals. Blood pressure was measured by the tail-cuff method. Kidneys were prepared for histological examination by standard methods. Renal function was determined by inulin clearance and urinary kallikrein by an amydolitic assay. RESULTS: In untreated FHR blood pressure, proteinuria and glomerulosclerosis increased with time. Captopril normalized blood pressure and stabilized proteinuria at pretreatment levels. At the end of the study, the incidence of glomerulosclerosis was significantly lower and comparable with the incidence at 5 months. Glomerular volume did not show a correlation with the incidence of glomerulosclerosis. Hemodynamic studies showed a significant increase of glomerular filtration rate in captopril-treated rats. No statistically significant effect was seen on renal plasma flow or filtration fraction. Urinary excretion of kallikrein was increased in captopril-treated rats. CONCLUSIONS: CEI is effective in protecting the kidney from structural damage in hypertensive FHR even when treatment is started under conditions of established glomerular injury. The protection given by captopril is probably related to intrarenal effects.