Activation of ERK1/2 implies the phosphorylation of tyrosine (pTyr) and threonine (pThr) by MEK1/2; both reactions were thought to be cytoplasmic， promoting ERK to reach the nucleus where it activates several transcription factors. In addition， H2O2 concentrations are known to modulate ERK intracellular translocation， which impacts on cellular proliferation. In this context， the objective of this work was to study the sequence of ERK phosphorylation under two redox conditions and to analyze a putative mitochondrial contribution to this process， in LP07 murine lung cells. A time-course of H2O2 administration was used and ERK phosphorylation was analyzed in cytosol， mitochondria and nuclei. At 1μM H2O2， a proliferative redox stimulus， immunoblot revealed a fast and transient increase in cytosol pTyr and a sustained increase in mitochondrial pTyr content. The detection for pThr/pTyrERK (2pERK) showed in cytosol a marked increase at 5 minutes with a fast dephosphorylation after that time， for both H2O2 concentrations. However， at 50 μM H2O2， an anti-proliferative condition， 2pERK was gradually retained in mitochondria. Interestingly， these results were confirmed by in vivo experiments using mice treated with a highly oxidizing agent [H2O2]. By the use of two ERK2 mutant constructions， where Tyr and Thr were replaced by alanine， we confirmed that 2pERK relied almost completely on pThr183. Confocal microscopy confirmed ERK subcellular distribution dependence on the incidence of cytosolic pTyr and mitochondrial pThr at 1μM H2O2. This work shows for the first time， both in vitro and in vivo， an ERK cycle involving a cross-talk between cytosol and mitochondria phosphorylation events， which may play a significant role in cell cycle progression， proliferation or differentiation under two different redox conditions.