Epithelial cells lining respiratory airways can participate in inflammation in a number of ways. They can act as target cells, responding to exposure to a variety of inflammatory mediators and cytokines by altering one or several of their functions, such as mucin secretion, ion transport, or ciliary beating. Aberrations in any of these functions can affect local inflammatory responses and compromise pulmonary defense. For example, oxidant stress can increase secretion of mucin and depress ciliary beating efficiency, thereby affecting the ability of the mucociliary system to clear potentially pathogenic microbial agents. Recent studies have indicated that airway epithelial cells also can act as "effector" cells, synthesizing and releasing cytokines, lipid mediators, and reactive oxygen species in response to a number of pathologically relevant stimuli, thereby contributing to inflammation. Many of these epithelial-derived substances can act locally, affecting both neighboring cells and tissues, or, via autocrine or paracrine mechanisms, affect structure and function of the epithelial cells themselves. Studies in our laboratories utilized cell cultures of both human and guinea pig tracheobronchial and nasal epithelial cells, and isolated human nasal epithelial cells, to investigate activity of respiratory epithelial cells in vitro as sources of cytokines and inflammatory mediators. Primary cultures of guinea pig and human tracheobronchial and nasal epithelial cells synthesize and secrete low levels of IL-6 and IL-8 constitutively. Production and release of these cytokines increases substantially after exposure to specific inflammatory stimuli, such as TNF or IL-1, and after viral infection.