Captopril (Capoten; Squibb) is a specific orally active antagonist of peptidyl-dipeptide carboxyhydrolase, the enzyme which converts angiotensin I to angiotensin II and which inactivates bradykinin. Captopril therefore reduces blood pressure in a variety of animal models of hypertension. In 96 studies on 1570 patients, captopril has been shown to be superior to placebo and equivalent to either propranolol or a diuretic in the treatment of essential hypertension. In the management of severe treatment-resistant hypertension, the response to captopril (alone or in combination with a diuretic and/or propranolol) was better than the response to standard triple therapy. Captopril, with digitalis and a diuretic, also improved the haemodynamic and clinical status of patients with refractory congestive heart failure. Side-effects include skin rashes (15%), proteinuria (1,1%, or 0,4% of patients with no prior renal disease) and the nephrotic syndrome (0,9%, or 0,3% of patients with no prior renal disease). Nearly all patients with the nephrotic syndrome in whom renal biopsies were performed were found to have membranous glomerulopathy. Neutropenia (total white cell count less than 1,000/microliter) was found in 33 of over 6,000 patients (0,4%), but in all cases there were other possible causes for this. Captopril is the first of an important group of antihypertensive and afterload-reducing drugs; its major indications are likely to be in the treatment of refractory severe hypertension or congestive heart failure.