The effects of angiotensin converting enzyme inhibition with captopril were investigated in patients with diabetic nephropathy and hypertension. After nine days' treatment with captopril glomerular filtration rate was unchanged in 13 patients, whereas renal plasma flow had increased from 265 to 302 ml/min/1.73 m2 body surface area (p less than 0.05) and the filtration fraction had decreased from 14.3 to 12.8% (p less than 0.025). During two years' treatment with captopril in 14 patients the mean arterial blood pressure had fallen by 5 mm Hg (p less than 0.005) and the deterioration in glomerular filtration rate had decreased from 10.3 to 2.4 ml/min/year (p less than 0.005). There was no correlation between the fall in blood pressure and the reduction in the deterioration of glomerular filtration rate. These findings suggest that the effects of angiotensin converting enzyme inhibition on renal haemodynamics protect renal function. Inhibitors of angiotensin converting enzyme should be considered for lowering blood pressure in patients with diabetic nephropathy.