Several hydroelectric dams in the Snake-Columbia river system are equipped with submersible traveling screens that project into the turbine intakes. The screens are designed to divert juvenile migrant Pacific salmon (Oncorhynchus spp.) and steelhead (Salmo gairdneri) from the intake upward into gatewells and the adjoining central bypass system. Assays of gill Na+,K+-ATPase were performed on yearling Chinook salmon (O. tshawytscha) collected during several routine fish guidance efficiency tests in 1985 and 1986. On three of the four sampling dates, gill Na+,K+-ATPase levels were significantly higher in fish guided into the gatewell than in those not guided. These data suggest there is a relationship between the physiological status of smolting yearling Chinook salmon and their susceptibility to guidance by traveling screens. Assessments of salmonid out-migrations may be biased if they are based on samples from traveling-screen guidance systems.