Long-term treatment of mature virgin female rats with a high dose of testosterone (24 weeks at a total dose of 300 mg) was associated with the development of endometrial adenomatous hyperplasia and resulted in a high incidence of adenocarcinoma when administered in combination with 7,12-dimethyl-benz[a]anthracene (DMBA; 2 mg in 0.1 ml sesame oil) introduced into the uterine cavity. Adenomatous hyperplasia occurred in all testosterone-treated rats, both with and without ovaries. Testosterone may promote the induction of endometrial tumors by DMBA.