The effects on cardiac output and portal venous pressure of atenolol, a cardioselective beta-blocker, and propranolol, a nonselective beta-blocker, were compared in patients with portal hypertension due to cirrhosis. One hour after p.o. administration of 100 mg of atenolol, cardiac output decreased by 32%, and portal venous pressure by 16%; the decrease in cardiac output and the decrease in portal venous pressure were significantly correlated. One hour after p.o. administration of 40 mg of cardiac output and the decrease in portal venous pressure were not correlated. Whereas the decrease in cardiac output was similar after atenolol or propranolol, the decrease in portal venous pressure was significantly less marked after the former than after the latter beta-blocker. It is concluded that: (a) the decrease in portal venous pressure determined by atenolol results mainly from reduction in cardiac output; (b) the decrease in portal venous pressure determined by propranolol results not only from reduction in cardiac output, but also from extracardiac effects of this beta-blocker, and (c) atenolol might be less efficient in the prevention of recurrent gastrointestinal bleeding in cirrhosis, since the decrease in portal venous pressure is less marked than that which occurs with propranolol.