The (re)uptake of 3H-5-HT was similar in hippocampal slices and synaptosomes from mice (C57BL/6), rats (Sprague DAwley), and macaque monkeys (Macaca fascicularis). In all species, the specific uptake was both saturable and showed high affinity for 5HT. However, the reuptake capacity was higher in rodents than in monkey. In a parallel series, radioautography was performed on hippocampal slices from the 3 species. The distribution of the reuptake sites extended into all hippocampal subfields, and a preferential relationship with pyramidal and granule cells was apparent. The innervation density followed mouse greater than rat greater than monkey. The innervation of the monkey Ammon's horn was the lowest of all fields examined. This anatomical observation of a decreased density in primate Ammon's horn was biochemically tested. Dissection of hippocampus into Ammon's horn and dentate gyrus showed much lower 3H-5-HT reuptake in the former. These results suggest a progressive decline in the serotonin innervation of the vertebrate hippocampus, with Ammon's horn more vulnerable than the dentate gyrus.