The study reported herein evaluates the influence of a chronic exposure to an urban pattern of NO2 (continuous baseline exposure of 0.2 ppm on which were superimposed two 1-hr spikes of 0.8 ppm NO2, 5 days/wk) as compared to the baseline exposure to determine the contribution of the spikes to toxicity. Mice were exposed for up to 52 wk with interim examinations. Multivariate analysis of variance revealed a statistically significant treatment effect on infectivity (p=0.05) and pulmonary function (p=0.03) parameters. Infectivity mortality of mice in the spiked exposure regimen was significantly greater than that in either the NO2-background exposed mice or in control mice. Four of the pulmonary function variables exhibited the greatest differences among the treatment groups: end expiratory volume, vital capacity, respiratory system compliance, and multiple-breath nitrogen washout. Results from the pulmonary function analyses indicate that the spiked exposures to 0.8 ppm NO2 may have induced a subtle lesion. The chronic study results indicate that the presence of spikes of NO2 is contributing significantly to effects on antibacterial lung defenses and pulmonary function of mice.