The purpose of this paper was to study the effect of sulpyrin on the absorption, excretion, metabolism, and protein binding of ampicillin in the small intestine of the rats and rabbits. The results are as follows; The absorption of ampicillin in small intestine of rats was increased by the combination of sulpyrin and ampicillin. The blood level of ampicillin in rabbits was elevated by oral administration of sulpyrin. The bioavailability of ampicillin was increased by simultaneous administration of sulpyrin and ampicillin. The urinary excretion of ampicillin was slightly decreased by combined administration of sulpyrin. The blood level of ampicillin was decreased and the urinary excretion was increased by long term administration of sulpyrin. On the other hand, metabolising enzyme of ampicillin was influenced by long term administration of sulpyrin. Protein binding rate of ampicillin was decreased by combination of sulpyrin as compared with control.