A comparative study between prazosin and propranolol with twenty male patients with mild to moderate essential hypertension was used to study the effects of these antihypertensive agents on plasma lipoproteins, and more specifically high density lipoproteins-2 and Apo A-I. After 4 weeks on placebo, the patients were randomly assigned to prazosin therapy (Group A) and other ten patients to propranolol therapy (Group B) for 8 weeks. Doses required for normalization of blood pressure ranged between 1 to 8 mg/day for prazosin and 20 to 240 mg/day for propranolol. The mean blood pressure was lowered to normal by both drugs. The results indicate that prazosin has no adverse effects on plasma lipids and lipoproteins separated by density gradient centrifugation. In contrast, propranolol increased total plasma triglycerides and decreased high density lipoprotein-2 apoprotein A-I and cholesterol levels by 50% in the group of ten patients. Since low levels of high density lipoproteins have been found to be associated with an increased incidence of coronary artery disease, the data obtained suggest that propranolol may induce significant, potentially atherogenic changes on lipid metabolism in hypertensive patients.