1. Cytoprotection by H(2)O(2) preconditioning against oxidative stress-induced apoptosis of PC12 cells has been demonstrated previously. In the present study, we investigated the effects of H(2)O(2) preconditioning on nuclear factor (NF)-kappaB activation and the role of NF-kappaB in the adaptive cytoprotection of H(2)O(2) preconditioning in PC12 cells. 2. The PC12 cells were preconditioned with 100 micromol/L H(2)O(2) for 90 min, followed by 24 h recovery and subsequent exposure to 300 micromol/L H(2)O(2) for a further 12 h. 3. The results showed that preconditioning with 100 micromol/L H(2)O(2) upregulated NF-kappaB expression and enhanced its nuclear translocation and DNA binding activity. In addition to its own effects on NF-kappaB expression, H(2)O(2) preconditioning also promoted the overexpression of NF-kappaB induced by a lethal concentration of H(2)O(2) (300 micromol/L). 4. N-Tosyl-l-phenylalanine chloromethyl ketone (TPCK; 20 micromol/L), an inhibitor of NF-kappaB, was administered 20 min before preconditioning with 100 micromol/L H(2)O(2). At this concenteration, TPCK blocked the overexpression of NF-kappaB induced by H(2)O(2) preconditioning, accompanied by attenuation of H(2)O(2) preconditioning-induced cytoprotection. The inhibition of NF-kappaB by TPCK enhanced caspase 3 activity induced by 300 micromol/L H(2)O(2). 5. The findings of the present study provide novel evidence for the effects of preconditioning with H(2)O(2) on constitutive activation of NF-kappaB, which contributes to the adaptive cytoprotection of H(2)O(2) preconditioning against PC12 cells apoptosis.