Increased metal concentrations have been associated with freshwater acidification. Continuous-flow acute toxicity tests were conducted in soft water to determine the effect of pH on the toxicity of cadmium, copper, and zinc to small (1-6 g) steelhead trout (Salmo gairdneri). LC50 values were calculated for 96- and 168-h exposure periods in waters of pH 4.7, 5.7, and 7.0. Test fish were significantly more tolerant of the metals at the lowest pH value than at higher pHs. The 96-h LC50 values at pH 4.7, 5.7, and 7.0 were 671, 97, and 66 micrograms/L for zinc, 66.0, 4.2 and 2.8 micrograms/L for copper, and 28.0, 0.7 and less than 0.5 micrograms/L for cadmium, respectively. The 168-h results were similar to the 96-h values. The results indicate that for the metals tested, toxicity is ameliorated in depressed pH waters over short exposure periods, such as may occur during snowmelt runoff. The possibility of hydrogen ion interference with metal uptake is postulated.