Concerns have been raised about the potential adverse effects on reproductive health in farm animals, humans, and wildlife species from a range of environmental chemicals that disrupt normal hormonal actions. The alkylphenol polyethoxylates are non-ionic surfactants used in the manufacture of detergents, paints and herbicides. During sewage treatment, these compounds are broken down to short chain alkylphenol polyethoxylates, alkylphenol carboxylic acids and alkylphenols which bioaccumulate in the lipid of living organisms. The estrogenic nature of one of these compounds - octylphenol has been clearly demonstrated in cell culture, in a recombinant yeast screen with human estrogen receptor?and in animal studies. It is proposed that these endocrine disrupting compounds influence male adult reproductive potential by disrupting the development of the hypothalamic-pituitary-testicular axis during fetal life. We have recently identified that exposure to octylphenol for the second half of gestation decreases circulating concentrations of FSH during fetal life and the number of Sertoli cells of the testis and testis size at birth in comparison to control animals (Sweeney et al., 2000). However, the testes size, % interstitial space, semen volume, semen concentration and % live semen was similar in both treatment groups in the adult. In contrast animals exposed to octylphenol from birth to weaning (16 weeks of age) had a significantly greater number of primary and secondary abnormalities in comparison to controls and animals exposed to octylphenol for the second half of gestation. A number of the animals exposed to octylphenol from birth to weaning exhibited augmented sexual behaviour, while those exposed to octylphenol for the second half of pregnancy showed a suppression of sexual behaviour. The current data suggests the physiological effect of exposure to octylphenol is dependant on the time and duration of exposure. This has major implications for the determination of universal end-point measurements to assess exposure to endocrine disrupting compounds.