An important component of the pathophysiologic response to hyperoxia (O 2 ) is pulmonary inflammation, although the roles of specific inflammatory mediators during pulmonary O 2 toxicity are not completely known. Interleukin-1 (IL-1) is an early inflammatory mediator and is sufficient to elicit many of the responses associated with acute injury. The IL-1 family comprises two bioactive proteins, IL-1α and IL-1β, and their natural antagonist IL-1ra. Here we report studies of IL-1 regulation during hyperoxic lung injury in the adult mouse. When assayed by Northern blot, increases in IL-1β mRNA were seen after 2 days of hyperoxia. In contrast, IL-1α mRNA was barely detectable before 4 days of hyperoxia. To further understand the cellular origin of IL-1β expression in lungs, in situ hybridization and immunohistochemical analyses were performed. IL-1β mRNA or protein was not detected in the lungs of unexposed animals. At 3 days, we observed the accumulation of IL-1β transcripts in pulmonary interstitial macrophages and in a subset of neutrophils, and immunodetectable IL-1β protein was co-localized in adjacent sections. At 4 days of exposure, IL-1β transcripts were widespread in lung tissue, but many areas rich in IL-1β mRNA were devoid of immunodetectable IL-1β. However, it is not known whether increased synthesis of IL-1β or the uncoupling of IL-1β protein and mRNA accumulation has a role in pathophysiology of pulmonary O 2 toxicity.