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‐h spikes of 0.8 ppm NO2, 5 d/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‐backgmund‐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 NO2may have induced a subtle lesion. The chronic study results indicate that the presence of spikes of NO2is contributing significantly to effects on antibacterial lung defenses and pulmonary function of mice.