Treatment of five hypertriglyceridemic and one normolipemic patients with propranolol, a non-selective beta-blocking agent, and metoprolol, which is generally considered to be a cardioselective beta-adrenoceptor antagonist, was observed to produce a further elevation in their triglyceride levels. The subjects were followed for 20 weeks, during which they received propranolol for eight weeks, a placebo for four weeks and metoprolol for eight weeks in a crossover design. The analysis suggested that the increase in triglyceride levels was less pronounced with the more cardioselective agent. Moreover, alpha-cholesterol decreased significantly after propranolol but remained constant while on metoprolol. Both drugs decreased beta-cholesterol whereas chylo- and pre- beta-cholesterol increased, particularly with propranolol. Considering the contradictory evidence regarding the clinical importance of chronic elevation of plasma triglyceride and decreased alpha- und beta-cholesterol concentrations, it is suggested that a cardioselective drug would be presently preferable in hyperlipidemic subjects.