The promoting effects of medium chain glyceride (MCG) on the rectal absorption of beta-lactam antibiotics were compared among various experimental animals (such as rats, rabbits and dogs). The plasma levels of cefmetazole (CMZ) after rectal administration, when MCG was used as the vehicle, significantly increased in all animals, but the bioavailability of CMZ was greatly varied among the animal species; the bioavailability of CMZ in dogs was apparently less than those in other animals (rats = rabbits greater than dogs). However, in dogs, the promoting effect of MCG on the rectal absorption of CMZ increased by addition of nonionic surfactants such as ether-type (Brij 30 and Brij 35) and ester type (Nikkol MYL-10). And the promoting effect of surfactants in the MCG vehicle is not always correlated with blood hemolysis, suggesting contribution of other physicochemical or biological factors.