The present study was conducted with the aim of investigating the effect of waterborne phytoestrogens on serum steroid hormones in common carp ( Cyprinus carpio ).To address that, 120 immature fish weighing 52-80 g were randomly divided into 5 groups of genistein, dissolved in water (15, 30, 60, 120, 240 µg/l) as well as a control group and were fed daily at the rate of 3% of body weight. With randomly selecting 7 fish from each group, blood sample was taken from fish peduncle at day 6 and 30 after of the exposure period. Thereafter, serum steroid hormones (progesterone, testosterone and estradiol), as well as some biochemical parameters (phosphorus, glucose, cholesterol, total protein, albumin, globulin, triglycerides and calcium) and hematological parameters were examined. The results showed that different concentrations of genistein had no effect on hematological parameters, but the amount of steroid hormones (i.e. progesterone, testosterone and estradiol) and some biochemical parameters (i.e. phosphorus, glucose and cholesterol) were affected. There was a significant decrease in serum progesterone concentration in 30, 60 and 120 µg/l genistein treatments compared to the control group at day 6 (P 0.05), whereas serum progesterone concentration significantly decreased in all genistein treatments at day 30(P 0.05). Serum testosterone levels decreased significantly in all treatments compared to the control group at day 6; also there was a significant decrease in serum testosterone levels of all treatments except for 15 µg/l genistein treatment at day 30 (P 0.05). Estradiol levels significantly declined in all treatments except for 15 µg/l genistein treatment at day 6 (P 0.05) and also there was a significant decrease in estradiol levels in all treatments at day 30 (P 0.05). With regards to biochemical parameters, serum glucose levels were significantly increased at day 6 in all treatment (P 0.05), while a significant increase in glucose levels was observed only in 120 and 240 µg/l genistein treatments at day 30. Serum cholesterol levels significantly decreased in 60, 120 and 240 µg/l genistein treatments at day 6 (P 0.05), while there was a significant increase in serum cholesterol levels at day 30 (P 0.05). Serum phosphorus levels were significantly decreased at day 6, whereas there was an insignificant increase in this rate at day 30. According to the results of this study, it seems that even low levels of phytoestrogens in water, can cause changes in steroid hormones levels, which can affect the sexual maturation and reproduction of common carp. Keywords: Biochemical parameter, Genistein, Hematological parameter
