Adsorption studies in vitro of mexilletine onto activated charcoal were performed in macrogol (polyethylene glycol)-electrolyte solutoin (PEG-ELS) and JP XII disintegration medium No. 2 (second medium). Mexiletine was adsorbed more extensively onto activated charcoal in PEG-ELS than that in JP XII second medium. The maximum adsorptive capacity of activated charcoal for the durg was 328 and 284 mg per gram of charcoal in PEG-ELS and JP XII second medium, respectively. In addition, the equilibrium constant of activated charcoal estimated according to the Langmuir equation was 0.079 and 0.034 l per gram of charcoal in PEG-ELS and JP XII second medium, respectively. Adsorption of mexiletine onto activated charcoal was decreased by omitting macrogol, sodium sulfate or sodium bicarbonate from a standard PEG-ELS formulation. Oral activated charcoal will be useful in combination with whole bowel irrigation with PEG-ELS in mexiletine overdose because of its excellent adsorbability in the solution.