Juvenile coho salmon (Oncorhynchus kisutch) were injected with one of two recombinant bovine hormones, growth hormone (bGH; 5.0 and 0.5 micrograms.g-1 body wt) or placental lactogen (bPL; 5.0, 0.5, and 0.05 micrograms.g-1 body wt) to determine the effect on growth, plasma cortisol concentration, cytosolic corticosteroid receptors (CR) in the gills, and the development of hypoosmoregulatory ability. One week following a single injection or six weekly injections of bGH or bPL, the fish were measured and sampled for CR concentration and Na+,K(+)-ATPase activity in the gills. Fish were also challenged with salt water (salinity 25%) for 24 hr to determine saltwater tolerance at the end of the 6-week treatment. Treatment with bPL and bGH significantly increased weight and length of the fish. The 0.05-micrograms bPL dose significantly elevated plasma cortisol concentration, whereas all other hormone treatments did not affect cortisol levels. bPL and bGH also significantly increased CR concentration and Na+,K(+)-ATPase activity in the gills. The perturbation in plasma sodium concentration was least in animals receiving the highest dose of bPL and the bGH-treated animals following transfer to seawater. An increase in cytosolic CR by bGH and bPL may increase responsiveness of the gills to cortisol and partially account for the increase in Na+,K(+)-ATPase activity and greater ability to regulate plasma sodium in seawater as exhibited by the experimental groups.