Prenatal tobacco smoke (TS) exposure has been implicated in various adverse health outcomes in the offspring, including poor development of lung and immune system, which in turn can alter the response of neonates to environmental challenges. This study was performed to determine whether in utero exposure to TS influences the pulmonary response of newborn rat pups to ozone (O₃). Timed pregnant Sprague-Dawley (SD) rats were exposed to TS or air for 3 h/d from gestation d 7 through 21. The pulmonary response of pups was assessed following a single 3-h exposure to air or 0.6 ppm O₃ on d 13 after birth. In all, 4 exposure groups were evaluated: (1) Air/Air (in utero air and postnatal air), (2) Air/O₃ (in utero air and postnatal O₃), (3) TS/Air (in utero TS and postnatal air), and (4) TS/O₃ (in utero TS and postnatal O₃). Bronchoalveolar lavage (BAL) was performed, and BAL cells and fluid were analyzed. Data revealed a significant increase in polymorphonuclear leukocytes (PMN) and total BALF protein in the Air/O₃ group compared to the Air/Air control, reflecting the inflammatory and cytotoxic effects of O₃. However, in utero exposure to TS attenuated PMN infiltration into the air spaces for recovery in the BAL of TS/O₃ pups. Lung tissue myeloperoxidase activity significantly increased only in the TS/O₃ group but not in Air/O₃ pups, thus suggesting that PMN are sequestered in the lung tissue and that the in utero TS likely inhibits O₃-mediated influx of PMN into the air spaces. Lung tissue analyses further showed a significant rise in manganese superoxide dismutase (SOD) protein and a decrease in extracellular SOD protein in the Air/O₃ group, suggesting oxidative effects of O₃. Interestingly, in utero TS exposure again suppressed these effects in the TS/O₃ group. Overall, results suggest that in utero exposure to TS alone produced minimal acute pulmonary effects in newborn rats, but modulated adverse effects of postnatal O₃ exposure. The results are contrary to the interactive toxic responses predicted for sequential exposures to TS and O₃, and may represent the development of "cross-tolerance."