Coho salmon smolts (Oncorhynchus kisutch) were transported at low and high densities. (12 and 120 g/L) for short and long periods (4 and 12 h). Because smolts can be transplanted directly to seawater, half of the fish in each treatment were transported to tanks containing seawater and half to tanks containing freshwater. Plasma corticosteroids and glucose were elevated at unloading in all groups. Corticosteroids were still above the resting levels 24 h later, whereas glucose had returned to basal levels at this time. Potential smoltification indicators such as plasma thyroxin concentration and gill Na-K-ATPase activity were not affected by transportation. Increased corticosteroids were correlated to increased mortality in transported salmon compared to acclimated control fish when subjected to a bioassay of stress severe confinement. It is concluded that transportation induced stress in the fish regardless of hauling regimen, that increased corticosteroids may have potential as indicators of reduced performance capacity, that the greatest stress occurred during loading and the first few hours en route, and that transported coho salmon smolts seem to be equally fit for entry into freshwater or seawater. Hormones of the hypothalamic-pituitary system probably mediate the environmental changes and endogenous rhythms that regulate the timing and physiological alterations of smoltification. Because thyroid hyperactivity is a major endocrine component of smoltification, yearling coho salmon were injected with mammalian prolactin (PRL) and thyrotropin (TSH) to determine their effect on plasma thyroxin concentrations. The response of plasma thyroxin to TSH is similar from January through May in coho salmon, suggesting that the thyroid does not change in sensitivity to TSH. A dose of 0.04 to 0.07 I.U. TSH is the minimum dose sufficient to significantly increase plasma thyroxin concentration. PRL (1 to 9 I.U.) depressed plasma thyroxin levels in coho salmon parr, smolts, and post-smolts. Increased plasma thyroxin and gill Na-K-ATPase levels tentatively are considered indicative of smoltification, migratory readiness, and hence, seawater adaptability. In an experiment to consider a methodology which could be implemented at a culture facility to enhance the survival of ocean-going smolts and perhaps mitigate losses due to stunting, an abnormality of smoltification, coho salmon parr were maintained for 3 wk in water supplemented with sodium or calcium salts. Prolonged residence in sodium - supplemented freshwater increased plasma thyroxin levels and tended to elevate gill Na-K-ATPase activity. In contrast, acute exposure (24 h) to 75% seawater halved plasma thyroxin levels in coho salmon parr. Gradual acclimation to increased ambient salinity may accelerate changes in, or enhance, plasma thyroxin levels and gill Na-K-ATPase activity, and thus potentially improve the growth and survival of outmigrating smolts and reduce losses due to stunting. Plasma corticosteroid levels were determined during smoltification and in response to mammalian PRL and TSH. The interrenal tissue, which synthesizes corticosteroids, becomes hyperactive during smoltification. Exogenous PRL and TSH have no effect on plasma corticosteroid levels at any time during smoltification. Plasma corticosteroid levels increase eight-fold between early April and late May in coho salmon, concurrent with increasing gill Na-K-ATPase and seaward migration. Generally, plasma levels of thyroxin and corticosteroids are related inversely. Thyroxin levels are maximum in early April, with the onset of silvering, and corticosteroids are at minimal concentrations at this time. Thereafter, thyroxin levels decline and corticosteroids increase.