1. The biliary excretion of 25-hydroxy-[3H]-vitamin D3 (25-(OH)-[3H]D3)-derived materials after the intravenous administration of 25-(OH)-[3H]D3 (1.25 nmol/100 g body weight) was studied during a period of 3 h in homozygous (icteric) and heterozygous (anicteric) Gunn rats. 2. The heterozygous rats excreted significantly more 25-(OH)-[3H]D3-derived materials than the homozygous Gunn rats (7.2 +/- 1.5% vs 3.1 +/- 0.5% of the administered dose, P < 0.025). 3. The lower biliary excretion of 25-(OH)-[3H]-D3-derived material in homozygous compared with heterozygous Gunn rats was not due to differences in the plasma 25-(OH)-[3H]D3 concentrations nor was it due to differences in the uptake of 25-(OH)-[3H]D3 by the liver since similar liver/plasma concentration ratios were observed in both groups of animals; it seemed, however, to be due to differences in the biliary transfer of 25-(OH)-[3H]-D3, as indicated by a lower bile/liver concentration ratio in homozygous than in heterozygous rats (1.39 +/- 0.23 vs 0.49 +/- 0.06, P < 0.05). 4. Moreover, samples of bile obtained from homozygous rats contained no beta-glucuronidase-sensitive 25-(OH)-[3H]D3-derived materials but contained significantly more intact 25-(OH)-[3H]-D3 than samples obtained from heterozygous Gunn rats (P < 0.05). After hydrolysis of bile obtained from homozygous and heterozygous Gunn rats with the enzyme beta-glucuronidase, the total amount of intact 25-(OH)-[3H]D3 recovered was found to be similar in the two groups of rats.(ABSTRACT TRUNCATED AT 250 WORDS)