Micropuncture and morphological studies were performed in three protocols assessing the renal hemodynamic and structural effects of angiotensin I-converting enzyme inhibitors (CEIs) in the progression of glomerular injury. In protocol I, rats were subjected to 5/6 renal ablation and received no therapy, enalapril (CEI), or triple-drug therapy (TRX) for 12 weeks. Control of systemic and glomerular hypertension with CEI resulted in prevention of glomerular capillary hypertension and protection against glomerular injury. Despite equivalent control of systemic BP, failure of TRX to control glomerular hypertension was associated with no protection against eventual proteinuria and glomerular sclerosis, values for these indexes being as abnormal as in rats receiving no therapy. In protocol II, rats were again subjected to 5/6 renal ablation and followed for 18 weeks. Early institution of CEI soon after ablation again prevented systemic and glomerular hypertension and largely limited glomerular injury. In a third group, enalapril therapy was delayed for 8 weeks after ablation until hypertension and proteinuria were established. Late institution of CEI resulted in prompt reduction in systemic and glomerular capillary hypertension and stabilization of glomerular disease. In protocol III, CEI was administered to normotensive, moderately hyperglycemic diabetic rats. A modest, 20-mm Hg reduction in systemic arterial pressure was associated with the normalization of glomerular capillary pressure and a striking reduction in the development of albuminuria and glomerular injury. These studies suggest that CEI effectively prevent glomerular capillary hypertension and thereby afford protection against glomerular injury in diverse models of progressive renal disease.