Streptozotocin-diabetic rats kept moderately hyperglycaemic by daily injections of ultralente insulin for 4-6 weeks (group DM) demonstrated a higher glomerular transcapillary hydraulic pressure gradient (delta P), glomerular plasma flow rate and single-nephron glomerular filtration rate (GFR) than was observed in age- and weight-matched non-diabetic controls (group C). This rise in delta P was prevented by therapy with the angiotensin I converting enzyme inhibitor enalapril (15 mg/l drinking water, group DM + E) even though glomerular hyperperfusion and hyperfiltration persisted. Fourteen months after induction of diabetes, animals in group DM displayed high levels of albuminuria and an increased incidence of focal glomerular sclerosis; treatment with enalapril maintained these parameters at levels which did not differ from those observed in group C. We conclude that prevention of glomerular capillary hypertension with enalapril therapy obviates functional and structural glomerular injury in experimental diabetes mellitus.