The effects of pituitary stalk section on anterior pituitary morphology were studied in 18 female rhesus monkeys. 4 animals were studied within 1–14 days after the operation, while the others were examined later (average 17.2 months). In 3 monkeys there was no necrosis of the anterior lobe, whereas in the remaining animals, an area of infarction averaging 20% was found. Using immunocytochemical techniques, we found that corticotropes and somato-tropes were still present following the procedure, and that the number of lactotropes increased markedly. In contrast, gonadotropes, although present in the pars tuberalis, were no longer demonstrable in the pars distalis 3 weeks after stalk section. These results show remarkable agreement between endocrine studies and morphological observations, and indicate a variable degree of dependency of the various pituitary cells on central nervous system influence