In rats, the concentrations of the acidic antiinflammatory drugs salicylic acid, acetylsalicylic acid, phenylbutazone, flufenamic acid and indomethacin in the glandular portion of the gastric mucosa were determined 30 and 60 min after oral or subcutaneous administration. In another series of experiments, solutions of the drugs were introduced into the ligated stomach and the concentrations in the mucosa and in the contents of the stomach were determined after 60 min. The ratio between the concentrations in the musoca and those in serum or gastric contents were much lower than expected according to the distribution by passive non-ionic diffusion. This apparent discrepancy may be explained as a result of a drug-induced damage to the mucosal cell allowing free diffusion of ionized drug across the cell membrane.