Regulation of pulmonary inflammation involves an intricate balance of both proand anti-inflammatory mediators. Acute lung injury can result from direct pulmonary insults that activate alveolar macrophages to respond with increased cytokine expression. Such cytokine gene expression is mediated in part via NF-B. IL-10 has been previously identified as an important endogenous anti-inflammatory cytokine in vivo on the basis of inhibiting NF-B activation; however, the mechanism of this inhibition remains incompletely defined. We hypothesized that IL-10 regulated NF-B activation in vivo via IK inhibition. A bitransgenic mouse that allowed for externally regulated, lungspecific human IL-10 over-expression was generated. In the bitransgenic mice, introduction of doxycycline induced lung-specific, human IL-10 over-expression. Acute induction of IL-10 resulted in significant decreases in BAL fluid neutrophils (48%, p=0.03) and TNF (62%, p<0.01) following intratracheal LPS as compared to bitransgenic negative mice. In vitro kinase assays showed this decrease to correlate to diminished lung IK activity. Furthermore, we also examined the effect of chronic IL-10 overexpression in these transgenic mice. Results show that IL-10 over-expression in lungs of mature mice increased the number of intrapulmonary cells the phenotype of which was skewed towards increased B220+/CD45+ B-cells and CD4+ T-cells and was associated with increased CC chemokine expression. Thus, regulated, lung-specific IL-10 overexpression may have a variety of complex immunologic effects depending on the timing and duration of expression.