The interaction between probencid and nalidixic acid was studied pharmacokinetically in rabbits infused with or without acidic soiution (5% NH₄Cl). The results were as fellows. The blood level and the area under the blood cencentration curve of nalidixic acid administered intravenously was elevated by coadministration of probenecid and more elevated in rabbits infused with acidic solution. Probenecid inhibited the urinary excretion of nalidixic acid in rabbits infused with adidic solution. Therefore, biological half-life of nalidixic acid was prolonged by coadministration of probencid.