The metabolism of l-bunolol, a new beta-blocking drug, was studied in man after single oral 3-mg doses of 3H-labeled compound. Absorption from the gut was rapid and virtually complete. Peak levels of bunolol and of dihydrobunolol, an active metabolite, were observed at 1 hr. Excretion of the administered radioactivity was mainly into the urine (78% in 4 days), with only 3% appearing in the feces. Bunolol, bunolol glucuronide, bunolol sulfate, dihydrobunolol, and dihydrobunolol glucuronide were identified and quantified in the plasma. These compounds represented 82% of the radioactivity in plasma at 30 min and 55% at 24 hr. Plasma half-lives (+/-S.D.) were estimated to be 6.1 +/- 0.3 hr for bunolol, 9.1 +/- 1.9 hr for bunolol glucuronide, 17.4 +/- 2.5 hr for bunolol sulfate, 7.1 +/- 0.5 hr for dihydrobunolol, and 7.7 +/- 0.8 hr for dihydrobunolol glucuronide.