A study was conducted to determine the effects of technical grade sodium pentachlorophenate (Na-PCP) on the early developmental stages of the steelhead trout (Salmo gairdneri). In an experiment where embryos were exposed to Na-PCP from fertilization to hatching, 100% mortality occurred within one week after fertilization at concentrations down to 300 ppb (g/l); within 24 hours posthatch, 100% mortality occurred down to 50 ppb of Na-PCP. Alevin dry weight at hatch was decreased by exposure to Na-PCP and hatching was delayed. In 5-day tests, alevins usually died within 24 hours at concentrations down to 200 ppb, but little mortality occurred at lower concentrations. Continuous exposure to Na-PCP from fertilization to complete yolk absorption produced 100% mortality at 40 ppb Na-PCP but little mortality at 20 or 10 ppb. However, in water containing 5 mg O2/l, 20 ppb Na-PCP was 100% lethal and at 3 mg O2/l, 10 ppb was 100% lethal. Little mortality occurred at these oxygen levels in the absence of Na-PCP. Oxygen consumption rates of alevins in 40 ppb Na-PCP was higher than those of control alevins. Exposure to Na-PCP reduced yolk utilization efficiency and growth. The bioenergetic data obtained in the study are consistent with the concept that PCP disrupts energy metabolism.