Different doses of glucagon and glucagon-like peptide (GLP) isolated from coho salmon, Oncorhynchus kisutch were tested in vivo and in vitro on juvenile coho and chinook (O. tshawytscha) salmon. Results obtained suggest an involvement of these peptides in the regulation of plasma glucose, plasma fatty acids, liver glycogen, and the hepatic enzymes: glycogen phosphorylase, pyruvate kinase, triacylglycerol lipase, and glucose-6-phosphate dehydrogenase. Metabolic effects were more enhanced in summer than either in spring or in autumn. GLP was less effective than glucagon in stimulating glycogenolysis in vivo. Salmon glucagon, especially in low concentrations, was generally more potent metabolically than mammalian (porcine/bovine) glucagon. The interaction between glucagon-family peptides and insulin seems to be different from the one described in mammals: glucagon and GLP either lowered plasma circulating levels of insulin or showed no effect. Only at the time of parr-smolt transformation did GLP slightly elevate plasma insulin levels in coho salmon.