A single dose of 20 mg/kg of cefsulodin [3-(4-carbamoyl-1-pyridiniomethyl)-7β-(D-α-sulfophenylacetamido)- ceph-3-em-4-carboxylate monosodium salt] was administered subcutaneously to mice, and intramuscularly to rats and dogs. The plasma and tissue levels reached the peak 15-30 minutes after administration. In mice and rats, no plasma levels were measurable 2 and 4 hours after administration. In dogs, the plasma levels were measurable 6 hours after administration. The level in the kidney of mice was slightly lower than the plasma level, while in rats and dogs, the level in the kidney was higher than the plasma level. The cefsulodin levels in the lung of rats and dogs were relatively high, and the level in mice was relatively low. The hepatic levels were very low in all test animal species. Cefsulodin was mainly excreted into the urine, and the excretion of cefsulodin into the bile was very slight.