The transcription factor NF-κB stimulates the transcription of proinflammatory cytokines including TNF-α. LPS (endotoxin) and hypoxia both induce NF-κB activation and TNF-α gene transcription. Furthermore, hypoxia augments LPS induction of TNF-α mRNA. Previous reports have indicated that antioxidants abolish NF-κB activation in response to LPS or hypoxia, which suggests that reactive oxygen species (ROS) are involved in NF-κB activation. This study tested whether mitochondrial ROS are required for both NF-κB activation and the increase in TNF-α mRNA levels during hypoxia and LPS. Our results indicate that hypoxia (1.5% O 2) stimulates NF-κB and TNF-α gene transcription and increases ROS generation as measured by the oxidant sensitive dye 2′, 7′-dichlorofluorescein diacetate in murine macrophage J774. 1 cells. The antioxidants N-acetylcysteine and pyrrolidinedithiocarbamic acid abolished the hypoxic activation of NF-κB, TNF-α gene transcription, and increases in ROS levels. Rotenone, an inhibitor of mitochondrial complex I, abolished the increase in ROS signal, the activation of NF-κB, and TNF-α gene transcription during hypoxia. LPS stimulated NF-κB and TNF-α gene transcription but not ROS generation in J774. 1 cells. Rotenone, pyrrolidinedithiocarbamic acid, and N-acetylcysteine had no effect on the LPS stimulation of NF-κB and TNF-α gene transcription, indicating that LPS activates NF-κB and TNF-α gene transcription through a ROS-independent mechanism. These results indicate that mitochondrial ROS are required for the hypoxic activation of NF-κB and TNF-α gene transcription, but not for the LPS activation of NF-κB.