Nadolol, a new nonselective beta 1 and beta 2 adrenergic blocking agent, has a plasma half-life of 17 to 23 hours. We studied 37 volunteers with stable angina pectoris who had five or more episodes of pain per week and who also had a 1 mm or greater ST segment depression 80 msec past the J point during a Bruce protocol treadmill test. An eight-week placebo controlled run-in period preceded double-blind randomization to nadolol administered once per day (17 patients) or identical appearing placebo for four weeks (20 patients), after which an exercise test was done. Diaries for pain episodes and nitroglycerin consumption were kept. Exercise tests were performed 24 hours after the last nadolol or placebo dose. Episodes of pain per week were reduced 59.8 percent after nadolol and 28.2 percent after placebo (P less than .01). Nitroglycerin consumption after nadolol was reduced 66.8 percent while after placebo it was reduced 36.2 percent (P less than .05). Resting and peak heart rates and peak rate-pressure products showed typical reductions due to beta-blockade 24 hours after nadolol compared with stability of these during placebo, all P less than .001. Exercise time after nadolol increased 42.2 percent, which was more than the 14.5 percent increase after placebo (P less than .05). Exercise work after nadolol increased 64.7 percent, greater than the 22 percent increase after placebo (P less than .05). Mean ST segment depression at end of exercise was little changed before and after treatment in both groups, reflecting consistency of effort. Improvement in symptoms and work capacity associated with nadolol significantly exceeded the placebo group responses. Unlike other available agents of this class, a single daily dose of nadolol produced therapeutically effective 24-hour beta-blockade in patients with disabling angina pectoris.