Effect of complex formation on the drug absorption from the rat small intestine by the perfusion method in situ was investigated. Apparent absorption rate of carbazochrome or nicotinamide alone was compared with that together its complexing drug. As the complexing drugs of carbazochrome, the following drugs were used : nicotinamide, 7-(2-hydroxyethyl) theophylline (HET), caffeine, 7-(2, 3-dihydroxypropyl) theophylline sodium (DHPT), and p-aminosalicylate (NaPAS). HET was used as the complexing drug of nicotinamide. It was found that the apparent absorption rate of carbazochrome was enhanced by complex formation with HET and caffeine at pH 3.0 and 4.7, and with NaPAS at pH 6.0, and that there was no significant difference in the apparent absorption rate between alone and together the complexing drug in nicotinamide-HET at pH 3.0 and 3.9, in carbazochrome-nicotinamide at pH3.0 and 3.9, and in carbazochrome-DHPT at pH6.0. From these absorption experiments and physico-chemical property for the complex, it is assumed that an extent of molecular interaction and the difference between the absorption rate of the drug and that of its complexing drug affect the absorption of the complex.