Whole-body autoradiography with 14C-labelled N-nitrosodibutylamine (NDBA) in pregnant Syrian golden hamsters on day 15 of gestation showed an uptake of radioactivity in the fetuses and a localization of metabolites in the tracheobronchial and nasal mucosa. Experiments in vitro showed a considerable capacity of these tissues of 15-day-old fetuses to form 14CO2 and tissue-bound 14C from the 14C-labelled NDBA, whereas in 12-day-old fetuses these transformations were insignificant. In infant hamsters (4-20 days old) the 14CO2-formation and the 14C-tissue-binding in the respiratory tissues were marked and in some instances even higher than in the tissues of adult animals. In hamsters the respiratory tissues are targets of transplacental carcinogenesis by NDBA and these tissues of the progeny may also undergo malignant transformations when the mothers are given NDBA during nursing. Our results indicate that a local bio-activation of the NDBA will occur in the respiratory tissues of fetuses at late gestation and in these tissues of infants and that this can be correlated to the carcinogenic effects.