Tricyclic antidepressants (TCA) bind to activated charcoal both in vitro and in vivo in healthy volunteers after a therapeutic dose of TCA. These findings provide a basis for the routine use of activated charcoal in TCA poisoning. The object of this study was to examine the effect of a single dose of 20 g of activated charcoal in overdose patients. Ninety-one patients from four centres with suspected TCA overdose were entered into a randomized study. Gastric lavage was performed on all patients. Thirty-four received 20 g of activated charcoal and 43 served as controls. Fourteen patients were excluded. Plasma drug concentrations were taken on admission and at 1, 2, 4, 8 and 24 h. The incidence of toxic symptoms was registered during 24 h. There was no significant difference in the area under the plasma drug concentration versus time curve, the peak plasma concentrations or plasma half-lives between the two groups. Toxic symptoms were more frequent in the non-treated groups although this difference was not statistically significant. In patients with TCA overdose initially treated with gastric lavage, a single dose of 20 g of activated charcoal had no effect on the systemic absorption or elimination of TCA.