The acute toxicities of arsenate, arsenite, cadmium, chromate, copper, mercury, silver, vanadium, and zinc were determined, individually and in two environmentally relevant mixtures, to two life stages of chinook salmon in reconstituted fresh and brackish waters that stimulated potential conditions in the San Joaquin Valley, California. The relative individual toxicities of the elements varied over four orders of magnitude; from most toxic to least toxic, the rank order was cadmium greater than copper greater than mercury greater than zinc greater than vanadium greater than arsenite greater than arsenate greater than chromate (no definitive tests for silver). In general, young fish tested in fresh water were more sensitive to the individual elements and the two mixtures than were advanced fry tested in brackish water. A 13-element mixture simulating concentrations in the San Luis Drain had the same toxicity to fish as the same mixture plus selenate and selenite, thus demonstrating that the presence of selenium at its environmental ratio in the mixture contributed no toxicity in short-term tests. As judged by a comparison of the individual acute values for salmon to the expected environmental concentrations in the San Luis Drain, the margins of uncertainty for cadmium and copper in both waters, zinc in fresh water, and mercury in brackish water were less than 100, whereas for the two environmental mixtures the margins were less than 15 in both test waters. These low margins of uncertainty, especially for the mixtures, indicate a high potential for environmental hazard to chinook salmon in the reconstituted waters tested.