A study was made of the histological changes occurring in the skin, stomach, liver, pancreas and kidney of adult gonadectomized male and female sockeye salmon in response to hormone treatments. The males received 11-ketotestosterone, 17α-methyltestosterone or cortisol for 4 or 7 weeks, and the females received estradiol, estradiol cypionate, or cortisol for 8 weeks.In the males, androgen injection evoked a highly significant increase in the thickness of the epidermis, a marked atrophy of the stomach, and a degeneration in the liver and kidney. In the pancreas, the exocrine portion showed characteristics of cytolysis in the acini, whereas the islets of Langerhans were hypertrophied. These changes were generally more pronounced after 7 weeks than after 4 but, in the skin, no further increase in the thickening of the epidermis was noted after the 4th week of treatment. No differences between responses of these tissues to 11-ketotestosterone and 17α-methyltestosterosne were detected.Estrogen administration in the females evoked similar, albeit weaker, responses in the skin, stomach, pancreas, and kidney than those recorded in the androgen-treated males. The liver of the females exhibited the characteristics of a hyperactive organ, which probably reflects estrogen-induced vitellogenesis.The effects of cortisol were similar in both sexes: little, if any, change was noted in the skin; atrophy or degeneration was observed in the stomach, liver, kidney, and in the exocrine portion of the pancreas, and the islets of Langerhans were hypertrophied. The cortisol-induced alterations were not, however, as pronounced as those noted in either the androgen- or estrogen-treated fish.These changes are discussed in relation to the changes observed in normal sexually maturing and reproducing Pacific salmon.