Male Sprague-Dawley rats were treated ip on Day 0 with 0, 20, 50, or 100 micrograms/kg of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and biliary tree permeability was evaluated on Days 2, 4, 7, 10, 14, or 20 by segmented retrograde intrabiliary injection of [3H]sucrose or [14C]mannitol. Seven days after 100 micrograms/kg TCDD, the percentage recovery in bile of both [3H]sucrose (73.9 +/- 4.2 vs 27.6 +/- 7.6, control vs TCDD, means +/- SE) and [14C]mannitol (22.7 +/- 2.2 vs 12.1 +/- 2.2) was decreased, demonstrating that the permeabilities of both the intracellular (canalicular) and paracellular pathways were increased. Seven days after 50 micrograms/kg TCDD, the recovery of [3H]sucrose was decreased (73.5 +/- 5.4 vs 39.0 +/- 2.8) but the recovery of [14C]mannitol was not (25.5 +/- 1.5 vs 22.9 +/- 1.9). Thus, an increase in paracellular permeability is obtained at a lower dose of TCDD. In rats treated with 100 micrograms/kg TCDD on Day 0, co-treatment with chlordecone (15 mg/kg/day on Days 2-6) or thyroxine (50 micrograms/kg on Day 2) had no effect. Pregnenolone-16 alpha-carbonitrile (75 mg/kg/day on Days 4-6) treatment increased [14C]mannitol recovery in both TCDD and control groups; its effect must not be specific. Methimazole given in drinking water (0.5%) on Days -7 through 7 reversed the increased permeability effects of TCDD (100 micrograms/kg) without affecting the biliary tree permeability ([14C]mannitol recovery) of control animals.