Various groups are concerned about metal releases to the aquatic environment via power plant effluents and their toxicity to organisms. Washington Public Power Supply System personnel performed acute and partial chronic flow-through bioassays of simulated cooling tower blowdown water to determine the effect on selected salmon species and life stages. Use of simulated water resulted in 96-h LC50 total copper values which ranged from 61–103 ug/L. The long-term (i.e., 60-day) no-effect level for total copper ranged from 18–24 ug/L. Water quality variation is believed to play a major role in establishing the toxicity of metals, and many individuals believe that it is a subcomponent of the total metal (e.g. dissolved, ionic/labile) which more closely correlates to the toxicity of a solution to fish. Our studies indicate that labile copper may be a more constant indicator of toxicity than the total metal for short-term effects.