Previous studies have indicated that teleost fish appear to have two somatostatin genes. In salmonid fish, it is purported that gene I encodes for somatostatin-14 (SS-14), while gene II encodes for somatostatin-25 (sSS-25). In the present study, the physiological effects of SS-14 and sSS-25 on carbohydrate and lipid metabolism in rainbow trout, Oncorhynchus mykiss, were evaluated by in vivo administration of hormone and measuring resulting levels of specific metabolites and hormones present within tissues and plasma. Somatostatin-14 administration caused hyperglycemia without affecting liver glycogen content and increased plasma fatty acid (FA) levels in association with enhanced activity of the lipid mobilizing enzyme, triacylglycerol lipase (TG lipase). Somatostatin-14 injection also resulted in reduced hepatic glucose-6-phosphate dehydrogenase activity, which may indicate a decrease in glucose channeling through the pentose phosphate shunt. In addition, SS-14 reduced plasma glucagon concentration, while having no effect on plasma insulin levels. Salmon SS-25 elevated plasma glucose levels in association with reduced glycogen content and resulted in increased plasma FA levels accompanied by increased hepatic TG lipase activity. Salmon SS-25 injection also resulted in a reduction in plasma glucagon and insulin levels. These results indicate that SS-14 and sSS-25 are important regulators of carbohydrate and lipid metabolism in rainbow trout and that modulation of metabolic activity by these peptides may be accomplished, in part, by alterations in insulin and glucagon levels circulating in the plasma.