The aim of this study was to investigate the toxic effects of pyrethroid pesticide lambda cyhalothrin in the presence of piperonyl butoxide as a modulator in the liver of juvenile Oreochromis niloticus. LC50 (96 h) value of lambda-cyhalothrin was determined as 2.901 mu g/L for O. niloticus. The fish were exposed to 0.48 mu g/L (116 of the 96-h LC50) lambda-cyhalothrin and 10 mu g/L piperonyl butoxide for 96-h and 15-d. tGSH, GSH, GSSG, Hsp70 and TSARS contents, GPx, GR, GST and caspase-3 enzymes activities were determined. Lambda-cyhalothrin caused increases in tGSH, GSH, TBARS contents, and GST activity. Piperonyl butoxide treatment with lambda-cyhalothrin caused significant increases in tGSH GSH, Hsp70, TBARS contents, and GPx and GST activities while caspase-3 activity was decreased. The results of the present study revealed that lambda-cyhalothrin caused oxidative stress which upregulated GSH and GSH-related enzymes. Piperonyl butoxide increased the oxidative stress potential and apoptotic effects of lambda-cyhalothrin. (C) 2012 Elsevier B.V. All rights reserved.