Although hemorrhagic shock causes a significant elevation of circulating levels of proinflammatory cytokines [tumor necrosis factor-α (TNF-α), interleukin (IL)-1β, IL-6], it remains unknown whether hypoxemia per se in the absence of blood loss activates macrophages (Mφ) to release increased amounts of these mediators. To study this, hypoxemia was induced in C3H/HeN mice by placing them in a plastic box that was flushed with a gas mixture containing 95% N2-5% O2 for 60 min, followed by return of the mice to room air. For control animals, the plastic box was flushed with room air. At 0, 2, or 24 h thereafter, blood samples were obtained, plasma was separated, and then peritoneal Mφ (pMφ) and Kupffer cells (KC) were isolated and incubated at 37°C for 24 h with lipopolysaccharide. The Mφ supernatants, as well as plasma samples, were assayed for TNF-α, IL-1β, and IL-6 with the use of specific bioassays. Hypoxemia induced a significant (P < 0.05) increase in plasma TNF-α levels during the entire study period while circulating IL-6 was elevated by 313% (P < 0.01) at 24 h after hypoxemia compared with shams. Moreover, the release of TNF-α, IL-1β, and IL-6 by pMφ and KC was markedly increased after hypoxemia compared with shams. Thus, hypoxemia in itself, in the absence of any blood loss or tissue injury, induces release of proinflammatory cytokines, which may contribute to systemic inflammation following hypoxemia.