Presmolt salmon, 16 months old and weighing 54.5 ± 1.2 g, were continuously exposed to 17-β estradiol by implanting a capsule containing the hormone. The fish and their non-hormone-treated controls were fed or starved. At the end of the third week the fish were killed. After hormone treatment the vitellogenin content of the fed fish was 37.9 ± 4.7 mg and of the starved fish 44.9 ± 3.3 mg/ml of serum. Hormone treatment decreased liver glycogen of the fed fish to 26 mg from 58 mg and of the starved fish to 11 mg from 28 mg/g wet weight of tissue. In muscle a significant decrease in glycogen was noted as the result of starvation. Acid proteinase activity per g wet weight of liver was unchanged. As compared with the starved controls, acid proteinase activity in liver per 100 g body weight was significantly elevated in all estradiol-treated fish. Myosin heavy chain content per g wet weight of muscle and per mg of DNA decreased as a result of hormone treatment. A diminished protein content in muscle after hormone treatment of both the fed and starved fish was paralleled by an increase in acid proteinase activity. In the fed fish, muscle aspartate amino transferase (EC 184.108.40.206) increased significantly after hormone treatment, whereas alanine amino transferase (EC 220.127.116.11), glutamate dehydrogenase (EC 18.104.22.168) and malic enzyme (EC 22.214.171.124) were not changed significantly. Estradiol treatment of the starved fish had no effect on the activities of the metabolic enzymes. Alanine amino transferase decreased significantly as a result of starvation. In conclusion, ingestion of food combined with hormone treatment decreased muscle protein content and increased acid proteinase and aspartate amino transferase activity. Food deprivation combined with hormone treatment followed a similar pattern but was without effect on the amino acid metabolizing enzymes. Muscle glycogen content decreased as a result of starvation. Hepatic glycogen content declined as a result of hormone treatment and starvation alone. The carbohydrates in liver appeared to be utilized for energy production during starvation and/or estradiol treatment. The amino acids derived from muscle proteins were made available by the stimulation of proteinases in hormone-treated fed and starved fish and they were utilized for protein synthesis or as energy source.