In the nematode Caenorhabditis elegans, stress resistance can be regulated by dauer formation (daf) genes. In the present study, regulation of heavy metal lead (Pb) toxicity by the 90-kDa heat shock proteins (Hsp90; daf-21) was investigated in both wild-type C. elegans and daf-21/Hsp90 mutants by focusing on the effects of varied temperatures below (15 degrees C) or above (25 and 30 degrees C) the presumptive optimum growth temperature (20 degrees C). More acute toxicity of Pb, indicated by the 24-h median lethal concentrations (LC50), was observed in wild-type adults than in the daf-21 mutant adults at 15, 20 and 25 degrees C; however, the daf-21 mutant adults showed more Sensitivity at 30 degrees C Enhanced Pb sensitivity (e.g., decrease LC50) in both types of C elegans was observed with both increased and decreased temperatures when compared to that at 20 degrees C. Additional examined endpoints included time course of toxicity at LC(50)s, pharyngeal pumping, reproduction, life span, and Hsp90 expression. Collective results showed that temperatures both above and below 20 degrees C exacerbated Pb toxicity, and that the protein level of daf-21/Hsp90 was one of the most sensitive indicators of Pb toxicity in wild-type C elegans, while pharyngeal pumping was more Pb sensitive in daf-21 mutants. Therefore, the expression of daf-21/ Hsp90 has apparent utility for the prediction and assessment of Pb-induced toxicity in nematodes. Further, the stress responses related to Hsp90 expression in C elegans may have considerable potential as sensitive biomarkers for the monitoring of environmental Pb contamination.