Migrating wild (W) and hatchery-reared (H) Chinook salmon (Oncorhynchustshawytscha) and steelhead (Oncorhynchus mykiss) juveniles were sampled after loading intofish-transport barges at Lower Granite Dam on the Snake River, Washington, and after bargetransportation downstream to Bonneville Dam on the Columbia River. Stress indices (increasedplasma cortisol and glucose concentrations and decreased plasma chloride concentrations) werehigher (P < 0.001) for Chinook salmon sampled during midseason (early to mid-May), when fishloading densities in barges were at seasonal maximums, than were stress indices for thosesampled earlier or later. Cortisol concentrations in Chinook salmon were correlated withsteelhead densities after loading of barges (P < 0.0001, R-2 = 0.41) and after arrival of barges atBonneville Dam (P < 0.0001, R-2 = 0.65). Cortisol concentrations were not correlated with gillNa+, K+-adenosine triphosphatase activities, which were higher in W than in H fish of bothspecies. Cortisol concentrations were higher (P < 0.0001 in 1994, P = 0.02 in 1995) in W than inH Chinook salmon, and concentrations declined in both groups during barge transportation earlyand late in the migration season but not during midseason. In contrast, cortisol concentrationswere lower (P < 0.001) in W than in H steelhead, were not correlated with steelhead loadingdensities, and declined in both W and H fish during barge transportation on all sampling dates.Electrolyte disturbances were greater in Chinook salmon than in steelhead, but disturbanceswere similar for W and H fish of both species. Stress-related water gain was, however, greater (orwas compensated more slowly) in W than in H fish. These results indicate that Chinook salmonare more stressed by barge transportation than are steelhead. If the viability of juvenile Chinooksalmon is reduced by adverse physiological, immunological, or behavioural responses totransportation stress, reductions in survival rates should be largest for fish transported duringmidseason, when densities of juvenile steelhead in the fish-transport barges are highest.