The influence of exposure of the airways to ozone on acute allergic responsiveness has been investigated in several species. Little is known, however, about the effect of this environmental pollutant on the late asthmatic response (LAR) in animals in which it is exhibited. The purpose of this study was to evaluate this effect in the canine peripheral airways and to assess the potential role of mast cells in modulating the effect. A series of experiments on seven mongrel dogs demonstrated that the numbers of mast cells at the base of the epithelial region of small subsegmental airways exposed to 1 ppm ozone for 5 min were significantly (p< .01) increased 3 h following exposure compared to air exposed or nonexposed control airways. In a second series of experiments performed on eight additional mongrel dogs with inherent sensitivity toAscaris suumantigen, antigen aerosol was administered to the sub‐lobar segment 3 h following ozone preexposure when mast cell numbers were presumed to be increased. These experiments were performed to determine whether ozone preexposure could enhance the late‐phase response to antigen by virtue of acutely increasing the number of mast cells available to bind the antigen. Four of the eight dogs tested displayed a late‐phase response to antigen following air‐sham preexposure. In these four dogs, simultaneous ozone preexposure of a contralateral lobe completely blocked the late‐phase response to antigen. These results indicate that the consequences of a single exposure to ozone persist beyond its effects on acute antigen‐induced bronchoconstriction and extend to the complex processes involved with the late response. This attenuating effect of ozone is seen under conditions where mast‐cell numbers in the airways are increased above baseline levels.