The effect of oral administration of activated charcoal on the clearance of theophylline and phenobarbital following their intravenous administration was studied in rats. Oral administration of multiple doses of activated charcoal significantly decreased the serum half-life and AUC (area under the curve) and increased the total body clearance of both theophylline and phenobarbital as compared with their respective controls. The volume of distribution was not significantly different between treatments. A single dose of activated charcoal showed only a slight enhancement of clearance of theophylline. Accelerated clearance of both drugs by oral activated charcoal was rationalized in terms of adsorption of exsorbed drugs and inhibition of their reabsorption by activated charcoal in the gastrointestinal tract.