The serum methotrexate (MTX) concentrations were estimated in two groups of patients after 6-hour iv infusion of 1 g/m2 of MTX and leucovorin rescue. One group (seven patients) received 25 g of activated charcoal orally at 12, 18, 24, 36, and 48 hours, while the other group served as a control (eight patients). Pharmacokinetic analysis revealed a significant reduction in the area under concentration curve from 18 hours until the serum MTX levels reached 10(-8) M. The MTX concentrations were significantly lower in the group that received charcoal than in the control group, at 18, 24, 36, 48, 60, and 72 hours from the end of infusion. First and second half-life values for the control and the charcoal-treated groups did not differ significantly. The data suggest that charcoal aids in the elimination of MTX from the body by interruption of the enterohepatic circulation of the drug. Activated charcoal did not cause any undesirable effects.