MtT/F84 grew well in Fischer rats (F344), but tumor growth was promoted in hyperestrogenized rats. Effects of dietary retinoic acid (RA) on tumor growth, estrogen receptor (ER) and serum growth hormone (GH) level were examined. Tumor latency became shortened, and tumor take and weight were promoted by all-trans RA at dosages of 50 and 200 mg/kg basal diet, but not dose-dependently. ER level was elevated in tumor of RA-treated rats, whereas the retinoic acid-binding protein level remained unchanged. RA also elevated incorporation of 5-bromo-2'-deoxyuridine, a thymidine analogue, into DNA of tumor cells. Average serum GH level was increased in tumor-bearing rats treated with RA and was well correlated with tumor weight. RA may directly affect ER level and enhance estrogenic action, resulting in promotion of tumor growth, or it may act independently for tumor growth and elevation of serum GH level.