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A 96-hour toxicity test with fish ventilation frequency as the response variable was developed to estimate safe zinc concentrations for steelhead Salmo gairdneri. Two Oregon steelhead strains were exposed to zinc at different water temperatures and total hardnesses (7, 12, and 17 C; 25 and 125 mg/liter as CaCOa). Ventilation frequencies enumerated from bioelectric potentials generated by buccal and opercular openings and closings showed significant increases at the highest test concentration in five of 10 tests. At 12 C and 25 rag/liter hardness, responses were detected at a zinc concentration of 144/zg/liter; the "safe" concentration determined in a chronic exposure of embryos and juveniles was between 444 and 819 /zg zinc/liter. This indicates that ventilation-frequency tests are at least as sensitive as long-term toxicity tests and may be used to screen chemicals for potential harmful effects on fish.This study was designed to determine the effect-no-effect concentrations of zinc for steelhead Salmo gairdneri based on ventilationrate monitoring. We developed a system to expose steelheads to zinc solutions in flowthrough chambers so that ventilatory patterns could be measured with external electrodes.The system was evaluated for two steelhead strains under varying regimes of temperature and hardness. Minimum zinc concentrations