This study investigated the effects of administering Estradiol Valerate (E2V) on gonadal sex differentiation in rainbow trout (Oncorhynchus mykiss, W., 1792). Rainbow trout fry of average weight 0.32 +/- 0.02 g, 65 days post fertilization (dpf), which had just started to feed were supplied with a feed containing E2V at a rate of 20 mg kg(-1) for 8 weeks (65-121 dpf). Fish samples were taken at random at 156 days (dpf,), 170, 191, 226 and 261 days and changes in fish gonads were analyzed histologically. At 156 dpf of the experiment, it was observed that oocyte formation had started in the fish gonads, suggesting feminization by the administration of the estradiol valerate. Intersex fish were observed at 170 dpf, while oocytes had gathered heads in gonads and sex transformation of some individuals had started at 191 dpf. Oocytes occupied the large areas of gonads and spermatogonia disappeared at 226 dpf as at 261 dpf. At the end of the experiment, sex compositions were 97% female, 3% intersex in the E2V administration group and 51% female, 49% female in the control group. There were no apparent differences between control group fish and hormone-treated fish as regards oocyte size.