A genetically male population of the common carp, Cyprinus carpio, was exposed to a measured concentration of 0.14 mg/L 4-tert-pentylphenol (TPP) for various periods before and during sexual differentiation in order to determine the presence of a labile period for endocrine disruption. Exposure to TPP for 3 days during the embryo-larval period did not affect the process of sexual differentiation nor the proliferation of the primordial germ cells. Longer exposures, starting before and including sexual differentiation, induced the formation of an oviduct. The oviduct was incomplete or completely closed depending on the timing of the exposure. Once present, this feature was persistent in individuals that were returned to clean water for 59 days. Exposures for various durations before or during sexual differentiation reduced significantly the number of primordial germ cells in a dose-related manner irrespectively of the timing of the exposure. These results demonstrate that the oviduct is a permanent feature that can be used as an end point in a test with male fish for detecting estrogenicity. In addition, they reveal that the period prior to sexual differentiation (24-51 days post hatch) is, besides the period sexual differentiation itself (50-110 days post hatch), sensitive to endocrine disruption, meaning that the duration of exposure of such a test could be reduced to a more narrow labile period.