This study was carried out to assess whether nadolol undergoes enterohepatic circulation. Eight healthy subjects received 80 mg nadolol orally on three occasions at least 2 wk apart. The first experiment was a control. The second consisted of nadolol followed in 3 hr by 3 gm activated charcoal given over a 9-hr period. In the third, the subjects received 0.5 gm erythromycin base and 0.5 gm neomycin four times a day orally for 2 days before nadolol. After the activated charcoal, the nadolol AUC fell from 2455 +/- 155 to 1355 +/- 123 ng . hr/ml (mean +/- SE), as did the percentage nadolol recovered in urine (15.4 +/- 1.4 to 10.2 +/- 0.7%) and the nadolol t1/2 (17.3 +/- 1.7 to 11.8 +/- 1.6 hr). These data suggest that nonrenal elimination increased. After the antibiotics, nadolol AUC was constant, percentage of nadolol recovered in urine fell to 12.7 +/- 1.7%, nadolol t1/2 fell to 11.6 +/- 1.3 hr, and mean peak nadolol concentration rose from 146 +/- 15 to 397 +/- 52 ng/ml. These results suggest that there is an enterohepatic circulation for nadolol, that activated charcoal may decrease nadolol bioavailability, and that antibiotics may increase the nadolol effect.