Macrophages exposed to hyperoxia in the lung continue to survive for prolonged periods. We previously reported (J. Biol. Chem. 278, 2003) that hyperoxia induces cell cycle arrest and sustained extracellular signal-related kinase (ERK) activity in macrophages. In this study, we determined the mechanisms of hyperoxia-induced ERK activation and how ERK activity plays a pro-survival role in hyperoxia-exposed cells. Inhibition of ERK activity decreased survival of hyperoxia-exposed macrophages. This was due, at least in part, to down-regulation of the pro-apoptotic Bcl-2 family member, BimEL. In determining the mechanism of ERK activation by hyperoxia, we found that ERK activation was not associated with hyperoxia-induced activation of the upstream ERK kinase, MEK1/2. When we examined the ability of whole cell lysates from hyperoxia-exposed cells to dephosphorylate purified phosphorylated ERK, we found decreased ERK-directed phosphatase activity. Two particular ERK-directed phosphatases (PP2A and MKP-3) demonstrated decreased activity in hyperoxia exposed cells. Moreover, whole cell lysates from normoxia exposed cells depleted of PP2A or MKP-3 were also less able to dephosphorylate ERK. These data demonstrate that in hyperoxia-exposed macrophages, sustained activation of ERK due to phosphatase down-regulation permits macrophage survival via effects on the balance between pro- and anti-apoptotic Bcl-2 family proteins.