We compare the ionic composition of ASTM reconstituted water with surface waters of the United States. The relative concentrations of major cations in surface waters are variable: Ca:Mg molar ratios range from 0.3 to 12.9, and Na:K molar ratios range from 1.4 to 600. However, the relative concentrations of these cations in ASTM reconstituted water are constant: the Ca:Mg molar ratio is 0.7 and the Na:K molar ratio is 21.3. Recent research has demonstrated that Ca is more important than Mg in modifying metal toxicity to salmonids. Hence, differences in the relative concentrations of these cations between surface waters and ASTM reconstituted waters may bias metal toxicity results. We also evaluate the potential for Na to modify Cu toxicity in salmonids. Sodium concentrations as high as 32.S mg/L did not influence Cu toxicity. These results indicate that ASTM reconstituted water may not be appropriate for metal toxicity tests.