In 19 healthy men aged 50 with untreated mild essential hypertension (WHO group I classification) randomized into two groups, treatment (18 weeks) with oxprenolol (n = 10) lowered HDL cholesterol by 11.4% (P less than 0.02) and cholesterol ratio (HDL cholesterol X 100/LDL + VLDL cholesterol) by 13.7% (P less than 0.05) whereas atenolol (n = 9) lowered HDL cholesterol by 16.5% (P less than 0.02) and cholesterol ratio by 19.2% (P less than 0.01). In the total material (n = 19) the reduction of HDL cholesterol correlated positively with initial concentration of HDL (r = 0.48, P less than 0.05). Increments of total triglycerides by 20.0 and 17.9%, respectively, for the two drugs and small changes in total cholesterol, LDL + VLDL cholesterol and uric acid were not significant. The HDL cholesterol lowering effect of oxprenolol and atenolol observed in the present study may have clinical importance since such metabolic side effects have been postulated to counteract the beneficial effect of blood pressure reduction on development of atherosclerosis and coronary heart disease in mild essential hypertension.