THE increase in the number of reports of abnormalities in male sex development in wildlife and humans coincided with the introduction of ’oestrogenic’ chemicals such as DDT (1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane) into the environment, Although these phenotypic alterations are thought to be mediated by the oestrogen receptor, they are also consistent with inhibition of androgen receptor-mediated events, Here we report that the major and persistent DDT metabolite, p,p’-DDE (1,1-dichloro-2,2-bis(p-chlorophenyl)ethylene), has little ability to bind the oestrogen receptor, but inhibits androgen binding to the androgen receptor, androgen-induced transcriptional activity, and androgen action in developing, pubertal and adult male rats, The results suggest that abnormalities in male sex development induced by p,p’-DDE and related environmental chemicals may be mediated at the level of the androgen receptor.