The "cobalt" variant of rainbow trout (Oncorhynchus mykiss) lacks most of the pars intermedia of the pituitary, and shows significant obesity with an enlarged liver and a fat accumulation in the abdominal cavity. Plasma levels of growth hormone, prolactin, and somatolactin were significantly lower in the cobalt variant than those in the normal trout. In contrast, plasma insulin level was four times higher than that in the normal. Plasma levels of total protein, free cholesterol, and triacylglycerol were higher in the cobalt, while those of glucose and fatty acids were not different from the normal levels. In the white muscle, red muscle, liver, and mesenteric fat, the cobalt showed higher contents of triacylglycerol than the normal fish. There was no significant difference in tissue contents of phosphatidylcholine between the two groups of the trout, except for that in the mesenteric fat, exhibiting significantly lower content than in the normal fish. Activity of triacylglycerol lipase in the liver in vivo was lower in the cobalt than that in the normal trout, while there was no significant difference between the two in the cultured liver slices. Desacetyl-alpha-MSH stimulated lipolysis of triacylglycerol similarly in the cultured liver slices from the normal trout and from the cobalt variant. Results from this study suggest that the lack of pars intermedia and the increased plasma level of insulin are involved in a depression of lipid mobilization and obesity in this variant of rainbow trout.