The absorption, distribution, detoxification, and excretion of pindolol were investigated in mice, rats, dogs, rhesus monkeys, and humans. The absorption was very good in all species; however, the quantitative composition of the final excretion products was unique for each species. Considerable interspecies variation was also apparent with respect to excretion patterns. The pharmacokinetics of pindolol in humans was investigated in single- and multiple-dose studies. All results were in agreement with a three-compartment model. The absorption was rapid and a first-pass effect of 12% to 25% of the dose (mean 20%) was calculated. The half-lives of the excretion of radioactivity were 3.0, 1.2, and less than 100 hours. The pharmacokinetic parameters obtained in normal volunteers were compared with corresponding values obtained in patients with hypertension, renal insufficiency, or liver impairment. Results in these patients were not significantly different. The bioavailability interaction was investigated with drugs often coadministered with a beta blocker. No drug-drug interaction was found with hydralazine, hydrochlorothiazide, coumarin, or aspirin. Pindolol coadministration resulted in a possible lowering of digoxin levels. The bioavailability of the tablet dosage forM proposed for marketing was compared to that of a solution and found to be optimal.