Copper accumulation and induction of metallothionein (MT) were investigated in the liver of juvenile Prochilodus scrofa exposed to copper (96 h-LC50) at pH 4.5 (LC50=98+/-0.8 microg Cu l(-1)) and pH 8.0 (LC50=16+/-0.5 microg Cu l(-1)) at 20 degrees C. No significant difference was found in copper accumulated in the liver at either pH. The ratio between copper accumulation in the liver after 96 h exposure and the copper in the water at pH 4.5 and 8.0 was 5.1 and 34.4, respectively. MT in the liver of fish exposed to copper was isolated by DEAE-Sepharose chromatography and identified by SDS-PAGE. No MT was identified in the controls. MT concentration was 126.8+/-21.5 and 167.4+/-25.6 microg g(-1) wet mass in fish exposed to copper at pH 4.5 and pH 8.0 (P>0.05), respectively. The results indicate that the pH of the water is a determining factor in copper toxicity, and the increased concentration of MT in the liver of P. scrofa exposed to copper suggests the possibility of using MT as a biomarker to evaluate the biological response of this species to copper contamination in aquatic environments.