Effects of surface active agents (ionic and nonionic) on intestinal absorption of drugs were studied on three method, i. e., (1) circulation with surfactants, (2) preperfusion with surfactant solutions, followed by circulation of drug solution free from surfactants, and (3) feeding experiments of surfactants solutions, followed by circulation of drug solution. Effects of surfactants were reviewed from two points. One is the effect due to interaction of drugs with surfactants and the other effect due to degeneration of the intestinal mucosa. It is concluded that the absorption of ionized species of drugs were more decreased than that of unionized species in degenerated intestinal mucosa, except for ionic interaction between drugs and surfactants.