To investigate the population-level effects of exposure to environmental endocrine disrupters, a mesocosm-scale study was carried out in which the reproductive performance of groups of free-spawning three-spined sticklebacks, Gasterosteus aculeatus, exposed as juveniles to a model estrogen, was assessed. Juvenile sticklebacks were exposed to ethinyl estradiol (EE₂) at measured concentrations of (mean ± SEM) 1.75 ± 0.37 ng L⁻¹ and 27.7 ± 1.08 ng L⁻¹ for 4 wk posthatch and then reared thereafter in pristine lake water until they reached adulthood. Exposure to the higher EE₂ concentration resulted in the occurrence of ovotestis among males, whereas no gonadal abnormalities were evident among males exposed to the lower concentration of EE₂. In addition, when spawning was allowed in the mesocosm environment, fewer nests were built per male, and fewer eggs were deposited per nest, in the group exposed to 27.7 ng L⁻¹. Males from this group also exhibited a less intense nuptial coloration than control males. In the group exposed to 1.75 ng L⁻¹ EE₂ posthatch, significantly fewer nests were built than in the control group. These results demonstrate that the timing of exposure to estrogenic contaminants, in developmental terms, is critically important. Short-term exposure to estrogens as juveniles can clearly influence reproductive performance as adults, despite all growth and development subsequent to the exposure period taking place in an estrogen-free environment. In addition, these results suggest that reproductive dysfunction can occur even in fish with no gross abnormalities in gonadal structure. This suggests that the absence of gonadal intersex is not a reliable indicator of the reproductive potential, or estrogen-exposure history, of fish populations or the only important factor involved in compromising the reproduction of estrogen-exposed fish.