Distribution of cortisol among its free and protein-bound fractions in rainbow trout (Oncorhynchus mykiss): evidence of control by sexual maturation.

1. Total cortisol concentrations did not differ in sexually matured female, male, and immature rainbow trout. 2. The per cent cortisol bound to a corticosteroid binding protein was greater in mature female fish (48.2%) than in mature male (16.0%) and immature fish (19.5%). 3. The mature female fish exhibited a lower percentage of free cortisol (21.8%) compared to mature males (44.8%) and immature fish (43.2%). 4. Parallel aspects of the teleostean cortisol binding-protein and the mammalian counterpart are compared and commented upon.