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In this study, the effects of modifying Ca (0.2−4 mM), Mg (0.05−3 mM), Na (0.75−5 mM), and pH (5.5−7.5) on the chronic toxicity of zinc to juvenile rainbow trout (Oncorhynchus mykiss) were investigated using standard 30-d assays in which survival and growth were monitored. Survival was observed to be a more sensitive end point than growth, and mortality mainly occurred during the initial stages of the exposure. This suggested that the mode of action of zinc toxicity was mainly of an acute nature. A review and analysis of existing literature demonstrated similar results for most other fish species investigated. Overall, up to a 30-fold variation of zinc toxicity was observed, as indicated by no observed effect concentrations varying between 32.7 and 974 μg of Zn L^-1. Increased concentrations of Ca²⁺, Mg²⁺, Na⁺, and H⁺ (within the tested ranges) resulted in a reduction of chronic zinc toxicity by a factor of 12, 3, >2, and …