in bony fishes such as the rainbow trout, oncorhynchus mykiss, glucocorticoid (gc) and estrogenic hormones are associated with a variety of physiological functions. most notable are effects of gcs which allow the animal to respond to stressful events (donaldson, 1981; clearwater and pankhurst, 1997; stouthart et al., 1998) and the actions of estrogens in the hormonal control of reproduction (donaldson, 1973; arcandhoy and benson, 1998; aiukwe and goksoyr, 1998). circulating levels o f gcs and estrogens are regulated largely by activation of the hypothalamic-pituitary-interrenal (hpi) axis (donaldson, 1981; stouthart et ai., 1998), and the hypothalamic-pituitary-gonadal (hpg) axis (fostier et al., 1983; blazquez et al., 1998; kah et al., 1997), respectively in the case o f the hpi axis, the initial stimulus for biosynthesis and release of cortisol, the primary plasnia gc, is corticotropic releasing hormone (crh). neurons o f hypothalamic origin project their axons to pituitary target cells, corticotrophs (peter et al., 1990). crh released onto corticotrophs stimulates release o f adrenocorticotropic hormone (acth) into circulation (baker et al., 1996; stouthart et al., 1998). acth serves as the final signal in this pathway by stimulating interrenal tissue to synthesize and release cortisol into circulation (chester-jones et al., 1969). in turn, the level of plasma gcs may provide a feedback signal to the hpi axis, via activation o f hypothalamic glucorticoid receptors (grs).