Epidemiologic evidence suggests that chronic stress may alter susceptibility to air pollution, but spatial confounding between these factors limits the utility of epidemiologic methods to disentangle these effects and investigate physiologic mechanisms. Clougherty et al. (p. 769) compared respiratory responses to concentrated ambient particles (CAPs) and examined biological markers of inflammation in a rat model of social stress. Stressed animals displayed higher average C-reactive protein, tumor necrosis factor-?, and white blood cell counts than did nonstressed animals. Among stressed animals CAP exposures were associated with respiratory frequency, lower flows, and lower volumes, suggesting a rapid, shallow breathing pattern. Conversely, increased inspiratory flows and greater minute volumes were observed in animals with elevated CAP exposures but without stress. The authors conclude that CAP effects on respiratory measures differed significantly by stress group. These results, which support epidemiologic findings that chronic stress may alter respiratory response to air pollution, may help elucidate pathways for differential susceptibility.