The effect of activated charcoal on gastrointestinal absorption of isoniazid (INH) was determined quantitatively in rabbits. The presence of activated charcoal resulted to about 40% reduction of INH bioavailability as was indicated by reduction in the AUC0-24 hour. The plasma half live (T1/2) and peak plasma concentration (Cmax) of the drug were reduced, while the apparent volume of distribution (Vd) and total clearance (Cl) of the drug were increased in the presence of activated charcoal. The results present an assessment of adsorptive capacity of activated charcoal for INH in vivo and as such could serve as a cheap, effective and readily available means of emergency treatment of INH poisoning.