Using a single cotyledon perfusion model, the placental transport of four H2-receptor antagonists, cimetidine, famotidine, nizatidine, and ranitidine, was determined and compared using normal term human and normal preterm baboon placentas. In both the human and baboon placentas, the transport of each agent was similar whether administered singly or in combination with the other drugs. Drug transport was the same in both directions, maternal-to-fetal and vice versa, indicating a lack of preferential transfer. The H2-receptor antagonists were transported at about 40% the rate of the freely diffusable reference compound, anti-pyrine. There were no significant differences between the human and baboon in any of the parameters of placental function evaluated. Placental glucose and oxygen consumptions, and lactate production were comparable in the human and baboon preparations. The transport and clearance of each of the H2-antagonists were similar in each species.