Nineteen patients with essential hypertension (EH) were studied as outpatients. After administration of chlorthalidone, 50 mg/day for 4 weeks, prazosin 1–4 mg/day (1.82 ± 0.33 nig/day) was added for a period of 12 weeks. Prazosin lowered supine blood pressure from 149.7 ± 2.85/102.0 ± 2.75 mm Hg to 128.2 ± 3.0/86.1 ± 1.04 mm Hg (p 4) from 10.03 ± 0.29 μg/ml to 10.85 ± 0.42 p.g/ml (p < 0.005). and decreased triiodothyronine (T4) from 36.65 ± 0.62% to 35.42 ± 0.56%, which was not significant. The free thyroxine index (FTI) increased slightly from 3.67 ± 0.12 to 3.83 ± 0.14 (p < 0.025). However, all values remained within the normal range for the laboratory. Serum cholesterol increased insignificantly. Triglycerides decreased significantly from 223.4 ± 50.6 mg/mg/dl to161.7 ± 29.0mg/dl(p < 0.05). Highdensity lipoproteins(HDL)increascdsignificantly from30.1 ± 2.1% 36.0 ± 3.06 (p < 0.025). Low density lipoproteins (LDL) decreased insignificantly and very low density lipoproteins (VLDL) decreased from 20.9 ± 3.44 to 16.3 ± 2.85 (p < 0.005). The cholesterol ratio increased from 45.51 ± 4.3 to 64.71 ± 10.7 ( +42.1%). These results indicate that, in patients with essential hypertension. prazosin is an effective antihypertensive agent and that it significantly increases HDL. decreases VLDL. and improves the cholesterol ratio