The small intestinal absorption of aspirin at pharmacological concentrations was studied in the unanesthetized rat by using a single-pass perfusion technique. The rate of aspirin absorption remained linear with its concentration (0.5 to 10 mM). Intestinal aspirin absorption increased as the concentration of hydrogen ion, sodium taurocholate, and ascorbic acid in the perfusate increased. Aspirin absorption did not change after ethanol addition. At pH 3.5 or 6.5, intestinal absorption of aspirin was greater than gastric absorption of the compound. Aspirin was not absorbed by the stomach at pH 6.5. These experiments indicate that aspirin can be absorbed to an appreciable extent in its ionized form by the small intestine but not by the stomach.