The exsorption of theophylline into the small intestinal lumen after intravenous administration of aminophylline was studied by in situ single-pass perfusion technique. As the concentrations of the drug in the serum and the bile juice were decreased, the exsorption rate of the drug into the perfusate decreased obeying the apparent first-order kinetics. The half-lives of the drug concentrations in the serum and the bile juice were 2.13 and 2.58 h for pH 6.0 isotonic phosphate buffer, and were 2.71 and 2.32 h for pH 8.0 isotonic phosphate buffer, respectively. The amounts of theophylline excreted in the perfusate and the bile juice were 12.08% and 0.17% of dose for pH 6.0 isotonic phosphate buffer, and were 13.81% and 0.20% of dose for pH 8.0 isotonic phosphate buffer. These results demonstrated that a considerable amount of theophylline was exsorbed into the intestinal lumen. The mechanism by which oral activated charcoal enhanced clearance of theophylline administered intravenously may be adsorption of the drug transported into the gastrointestinal tract by the charcoal.