While much research has focused on the effects of individual chemical exposures on animal health, far less is known about the effects of exposure to the mixtures of chemicals often found within our environment, even though this is a more typical exposure pattern. Biosolids (processed human sewage sludge) contain low individual concentrations of an array of contaminants including heavy metals and organic pollutants [e.g., polycyclic aromatic hydrocarbons, polychlorinated biphenyls and polychlorinated dibenzo(p)dioxin and furan] and form the basis of our model with which to study the effects of exposure to mixtures of environmentally relevant concentrations of pollutants in a domestic animal, the sheep. Studies using this model have investigated the effects of developmental exposure to biosolids on a variety of reproductive endpoints, including GnRH, kisspeptin, and estradiol receptor expression within the hypothalamus, LH and estradiol receptor expression within the pituitary gland, and protein, mRNA, and gamete production within the gonads of male and female sheep. The studies suggest that exposure to biosolids in utero, via maternal exposure, has detrimental effects on the fetal hypothalamo-pituitary-gonadal axis that could affect subsequent fertility. Studies with adult animals, also exposed during fetal life, suggest long-term effects of environmental chemical exposure on the reproductive axis. Investigation of lambs born to ewes grazed on biosolid-treated pastures (1) throughout life, (2) up until gestation, or (3) only during gestation have shown that some effects of environmental chemicals (relative to unexposed controls) may be more pronounced when exposure is acute, or that physiological compensation may occur when exposure is prolonged. Overall, the results of this study question the reliance on no observed adverse effects levels, with regard to chemical safety, when chemical exposure normally occurs as complex mixtures. The results suggest that developmental chemical exposure may affect the hypothalamo-pituitary-gonadal axis at a variety of levels, although whether the effects are driven by central effects or occur at each organ studied remains to be determined. [Wellcome Trust grant 080338].