Adenohypophysial cell types and neurohypophysial tissues were investigated in the cobalt variant of the rainbow trout, which possesses an irregularly-shaped pituitary. The pituitary remnant was completely detached from the hypothalamus in all but one fish, in which a remnant was associated with the hypothalamus. Prolactin (PRL) and growth hormone (GH) cells were predominant cell types in all pituitary remnants, forming PRL and GH areas, respectively. There were fewer somatolactin (SL) and melanophore-stimulating hormone cells than in normal fish. There were few corticotropin cells in cobalt and normal trout. Although aldehyde-fuchsin positive cells were also present, positively-staining fibers were not detected in any of remnants examined. Plasma SL levels were much lower in cobalt than in normal trout. Plasma levels of GH and T3 were significantly lower in cobalt than in normal fish. Plasma levels of PRL, T4 and cortisol, plasma osmolality, ions (Na+, K+, Ca2+, and Mg2+), glucose, triglyceride, free fatty acids, and amino nitrogen concentrations were similar in the two groups. Cobalt variants thus lack hypophysial pars intermedia and neurohypophysial tissues. Various abnormalities of the cobalt variant, such as the cobalt blue body color and the fat deposition in the abdominal cavity, may be related to the absence of the pars intermedia.