The absorption of Aminopyrine from the small intestine of a rat in combination with antihistamic agent, Diphenhydramine, Pheniramine, Tripelennamine and Diphenhydramine respectively, was examined. Through the rat small intestine canal, a definite quantity of a sample solution comprising 0.5 mM of Aminopyrine and 0.1 mM of each antihistamic agent in phosphate buffer solution (pH=7.4) was perfused through the rat small intestine at rate 5 ml per minute. The samples of the circulating solution were taken out after 5 minutes of the perfusion to give initial concentration and every 30 minutes for 3 hours. The amount of residual Aminopyrine in the solution was determined photometrically at 720 m.mu. by using 0.4% potassium ferricyanate solution and 1% ferric nitrate in 0.1N nitric solution. The mixed compounds of Aminopyrine with antihistamic agent, such as Aminopyrine with Diphehydramine, Aminopyrine with Pheniramine, Aminopyrine with Tripelennamine and Aminopyrine with Chloropheniramine showed more increased absorption than Aminopyrine alone. the absorption rate constants and apparent permeability coefficients of the mixed compounds were shown in detail.