Angiotensin-converting enzyme inhibitory therapy is widely used to treat hypertension. With long-term use, it is now being shown to have a beneficial effect on renal function and proteinuria in patients with renal insufficiency. When hypertensive patients with renal insufficiency are treated with enalapril, glomerular filtration rate is maintained, effective renal plasma flow is increased, and microalbuminuria and gross proteinuria are reduced. These beneficial renal changes with enalapril therapy differ from those of most other conventional antihypertensive medications. Clinical awareness of potential problems with hyperkalemia and increasing azotemia, particularly in the setting of salt/volume depletion, is important to assure optimal patient management. When these problems occur, they are nearly always reversible by correcting salt/volume status and/or interrupting enalapril therapy.