We studied the pharmacokinetics and beta-blocking effects of a single, oral 20 mg dose of timolol in six poor metabolizers (PMs) and six extensive metabolizers (EMs) of debrisoquin. The plasma timolol concentration was significantly higher in PMs than in EMs. There was a fourfold difference in mean AUC (1590 +/- 1133 vs. 394 +/- 239 ng X hr/ml; P less than 0.01) and a twofold difference in mean t1/2 (7.5 +/- 3 vs. 3.7 +/- 1.7 hours; P less than 0.01), reflecting differences in oral clearance (13.1 +/- 7.8 vs. 48.5 +/- 23.2 L/hr; P less than 0.01). The degree of beta-blockade was greater in PMs than in EMs at 12 hours (30.9% vs. 18.2%; P less than 0.05) and at 24 hours (28.3% vs. 13.1%; P less than 0.05). In the group as a whole the metabolic ratio correlated positively with both kinetic data and beta-blockade, but some overlap was observed. Hence timolol metabolism appears to be subject to debrisoquin-type polymorphism, which results in interphenotypic variation in plasma concentration and beta-blocking effect.