An attempt to reveal endocrine disruption in bluegill fish (Lepomis macrochirus) upon exposure to diazinon was made. Fish blood serum was collected and estradiol levels were measured following times of exposure to 60 microg/L of diazinon. Samples were drawn from the fish during each exposure period of 24, 48, 72 and 96 h and 1 and 2 weeks. The sample estradiol levels were compared to those of a control. Ovarian tissue was examined. As necrosis increased within the ovaries, estradiol blood serum levels altered from those of the control sample (101 pg/mL). Histopathology within the follicles became apparent as exposure time to diazinon increased. The changes in the ovarian follicles can be correlated to the estradiol levels in the blood. Mean estradiol level for the control group was 101.53 pg/mL. Estradiol levels from the trial groups varied from those of the control (24 h, undetectable; 48 h, 74.04 pg/mL; 72 h, 57.16 pg/mL; 96 h; 39.15 pg/mL; 1 week, 66.50 pg/mL; 2 weeks, undetectable). It seems that the initial shock experienced by the fish after 24 h of exposure resulted in an undetectable level of estradiol. The level of estradiol initially ascended, but it began to decline and after 1 week of exposure it ascended a little and became undetectable after 2 weeks. Nevertheless, throughout the exposure period the level of estradiol was lower than normal. Severe damage to the mature oocytes may be the reason for undetectable levels of estradiol after 2 weeks of exposure.