Exposure to estrogenic chemicals during critical periods in fetal life can alter the development of reproductive organs, the neuroendocrine system, and subsequent behavior. We examined the effects of prenatal exposure to the estrogenic chemicals, o,p'-DDT (the estrogenic contaminant in commercial DDT) and the drug diethystilbestrol (DES), as a positive control, on different forms of aggressive behavior in both male and female house mice. We also examined effects of these chemicals on male reproductive organs. From gestation days 11-17 female mice were fed an average concentration (dissolved in oil) 0.018 and 0.18 ng/g body weight of DES. Doses of o,p'-DDT were 18 and 180 ng/g body weight, based on the prediction that the in vivo potency of o,p'-DDT would be approximately 1000-times lower than DES. We found that prenatal exposure to DES increased the frequency of both males and females that responded aggressively to a same-sex conspecific. Preputial glands in males exposed to the 0.018 ng/g dose of DES were significantly enlarged relative to controls. Males exposed to the 18 ng/g dose of DDT had smaller testes than controls. The possible implications of perturbing the development of social behaviors, such as aggression, on individuals reproductive success and social structure of the population are discussed.