Half of the offspring from nicotine exposed and control mother mice were cross-fostered at birth to form four groups of those exposed to nicotine only during the gestational period (NC), only during the lactational period (CN), throughout both periods (NN), or exposed to none (CC). Nodular mucosal bulges (neuroepithelial bodies, NEB) were found in all groups; their number decreased with the increase in the age in the control group (CC). The bulges were large and irregular in all nicotine exposed groups, especially in the NN group. Although they also decreased in number with age, NEB were significantly retained and abnormal in the NN group compared with the NC and CC groups on day 30 (p less than 0.01). In addition, the bronchioles were tortuous in the nicotine exposed groups probably due to the expansion of mucosal depressions, pits, and fenestrations. In mice, maternal exposures to a small dosage of nicotine throughout both gestational and lactating periods appear to induce more baby lung changes than when the exposure to nicotine is limited to either period alone. More importantly, nicotine induced baby lung changes observed on day 5 appear to regress by day 30, if the exposure to nicotine is discontinued at birth.