A patient with pancreatic carcinoma and a choledochal T tube was given tritiated vincristine sulfate ([3H]-VCR) intravenously. Peak biliary excretion occurred in 2 to 4 hr and this sample contained 9.7% of the injected radioactivity. The first 24-hr bile sample contained 21.7% of the dose and 76.4% of the cumulative 72-hr biliary excretion. During a 3-day period of observation, 4.2%, 45.6%, and 49.6% of the excreted radiolabel was present in the feces, urine, and bile, respectively. Products of VCR metabolism and decomposition appeared in the bile rapidly; only 46.5% of the drug was present in the parent form in the 2-hr collection. Since significant amounts of these products were also identified in control specimens of bile, blood, plasma, and buffer alone after brief periods of incubation, the origin and nature of the species appearing in the bile remain unclear. Observing the fecal route to be the major source of elimination of radiolabel following intravenous injection of [3H]-VCR in our patients previously, we now conclude the biliary system to be the principal route of excretion of VCR and its products. Hepatic dysfunction might therefore alter elimination kinetics and increase the exposure to VCR and its products which might augment toxicity and require dose modification.