Adult C. dimerus were exposed for 2 months to endosulfan (ES) during summer or winter. Pituitaries were processed for Western blot (WB) or immunohistochemistry (IHQ) to evaluate LH and FSH content. WB showed a decrease in the pituitary ir-FSH content and an increase in that of LH in exposed fish during summer, correlating with the morphometric analysis of ir-cells. In winter the opposite tendencies were observed in pituitary content. This supports the role of ES as an endocrine disruptor and the existence of seasonal variations in gonadotropin’s responsiveness to ES.