Brook trout were exposed to experimental water concentrations of methylmercuric chloride at concentrations from 0.01 to 2.93 micrograms of Hg/liter, to cadmium chloride from 0.06 to 6.35 micrograms of Cd/liter, and to lead nitrate from 0.90 to 474 micrograms of Pb/liter. Exposures were for 2- and 8-week periods. Fish weight and length, hemoglobin, and blood plasma sodium, chloride, glucose, glutamic oxalacetic transaminase, and lactic dehydrogenase were measured. The following observed changes were statistically significant: increases in plasma sodium and chloride and decreases in hemoglobin and glutamic oxalacetic transaminase activity caused by lead; increases in plasma chloride and lactic dehydrogenase activity and a decrease in plasma glucose caused by cadmium; and increases in hemoglobin and plasma sodium and chloride caused by methylmercury. Comparisons were made between these biochemical findings and published data from bioassay and tissue-residue studies regarding the determination of threshold indices of toxicity.