Habitual marijuana smoking is associated with inflammation and atypia of airway epithelium accompanied by symptoms of chronic bronchitis. We hypothesized that tetrahydrocannabinol (THC), the primary psychoactive component of marijuana, might contribute to these findings by impairing cellular energetics and mitochondrial function. To test this hypothesis, particulate smoke extracts from marijuana cigarettes, tobacco cigarettes and placebo marijuana (0% THC) cigarettes were examined for their effects on the mitochondrial function of A549 cells in vitro. Only extracts prepared from marijuana cigarettes altered mitochondrial staining by the potentiometric probe JC-1. Using a cross-flow, nose-only inhalation system, rats were then exposed for 20 min to whole marijuana smoke and examined for its effects on airway epithelial cells. Inhalation of marijuana smoke produced lung tissue concentrations of THC that were 8-10 fold higher then those measured in blood (75 ± 38 ng/g wet wt tissue vs. 9.2 ± 2.0 ng/ml) suggesting high local exposure. Intratracheal infusion of JC-1 immediately following marijuana smoke exposure revealed a diffuse decrease in lung JC-1 red fluorescence compared with tissue from unexposed or placebo-exposed rats. Exposure to marijuana smoke in vivo also decreased JC-1 red fluorescence (54 % decrease, p < 0.01) and ATP levels (75 % decrease, p < 0.01) in single cell preparations of tracheal epithelial cells. These results suggest that inhalation of marijuana smoke has deleterious effects on airway epithelial cell energetics that might contribute to the adverse pulmonary consequences of marijuana smoking.