SUMMARY-We studied the endocrine status of SJLjJ mice. Light and electron microscopy revealed that the adenohypophyses of both sexes became progressively infiltrated with an abnormal number of gonadotropinproducing cells that probably secreted large amounts of luteotropic hormone. The ovaries had numerous large corpora lutea even in animals over 1 year of age with reticulum cell neoplasms. The adrenal cortexes of female mice showed no regression of the reticular zone. In accordance with the anomalous condition of the adenohypophysis and ovary, females had abnormal estrous cycles, with prolonged diestrus and consequent reduction in fertility. These data were discussed in the context of hormone environment versus onset of systemic neoplastic disease and the relationship between hormone dependence and leukemic virus expression.-J Natl Cancer Inst 53: 731-744,1974.