Presmolt Atlantic salmon (Salmo salar), 16–17 months old and weighing 65 ± 5 g, were fed either a control diet or a diet containing equal portions of pyruvate and α-ketoglutarate replacing 10% of the total feed dry weight. In a pilot experiment the keto acid diet had been fed to 5-month-old salmon during 5 weeks without adverse effects on growth and the body amino acid content. Nitrogen excretion was markedly diminished. In the major experiment the 16–17-month-old presmolt salmon were divided into four groups. After one week on the diets one control and one keto acid group were continuously exposed to 17-β-estradiol by implanting a capsule containing the hormone. After further 4 weeks the fish were killed. The vitellogenin concentration in the plasma was 50.5 ± 5.1 mg and 56.7 ± 5.2 mg/ml for the hormone treated fish fed the control and the keto acid diet, respectively. Liver glycogen content decreased during hormone treatment, but the fish fed on the keto acids were less affected. Upon exposure to estradiol the activity of tyrosine aminotransferase (EC 2.6.1.5) and malic enzyme (EC 1.1.1.40) was diminished in both feeding groups. Phosphoenolpyruvate carboxykinase (EC 4.1.1.32) was elevated in activity after feeding the keto acid diet but was unaffected by hormone treatment. Isocitric dehydrogenase (EC 1.1.1.42) and glutamate dehydrogenase (EC 1.4.1.3) were elevated in activity after hormone treatment of the fish fed on the keto acids. Fatty acid synthetase (EC 2.3.1.38) as well as glucose-6-phosphate dehydrogenase (EC 1.1.1.49) plus 6-phosphogluconate dehydrogenase (EC 1.1.1.43) were unchanged by dietary and hormonal conditions. The high vitellogenin synthesis in the liver was reflected in a decrease in protein content per g wet weight of muscle. Proteins of sarcoplasmic origin were more affected by estradiol treatment than those of myofibrillar origin. No differences between the dietary conditions were noted. Muscle glycogen content was elevated in the fish fed on the keto acids without an effect by estradiol. In summary, a utilization of dietary pyruvate and α-ketoglutarate in carbohydrate metabolism seemed to occur. Pyruvate and α-ketoglutarate can be used as an alternative carbohydrate source in the diet. The advantage of such a diet is the diminished nitrogen excretion by the fish into the water without a negative effect on growth and vitellogenin synthesis.