The present study was designed to determine if neonatal exposure to the estrogenic mycotoxin zearalenone or the weakly estrogenic pesticide methoxychlor could masculinize and/or defeminize the behavior of female hamsters. Neonatal hamsters were given a single s.c. injection of either zearalenone (1mg/pup), methoxychlor (1mg/pup), 17 Beta 0estradiol (E2) (40 micrograms/pup) or the vehicle two days after birth. After puberty, behavioral estrous cyclicity was measured. The females were then ovariectomized, treated with the male hormone testosterone, and tested for their ability to mount a receptive female (a behavior not normally displayed by female hamsters). Females treated neonatally with estradiol or zearalenone were masculinized but not defeminized, an effect consistent with perinatal exposure to low doses of sex hormones. Females in these two treatment groups displayed normal four day behavioral estrous cycles, but following ovariectomy and testosterone treatment they mounted a sexually receptive female at levels comparable to the males. Methoxychlor-treated females did not differ from controls.