Stimulation of [3H]thymidine incorporation in the lungs of hamsters exposed singly and repeatedly to 10 ppm NO2was studied. Within 24 hr of the first exposure to NO2, the response was characterized by a marked increase in [3H]thymidine labeling in the bronchi, bronchioles, and alveolar ducts, but not in the trachea or the peripheral alveoli. If, after the first NO2exposure, subsequent exposures were repeated daily, no further stimulation of [3H]thymidine incorporation was observed in any part of the respiratory tract. However, if the interval between the first and subsequent exposures was 2–3 days rather than 1 day, stimulation of [3H]thymidine incorporation was observed in the bronchi, bronchioles, and alveolar ducts for up to 21 NO2exposures. A significantly greater increase in [3H]thymidine incorporation was observed when the interval between subsequent exposures was extended to 7 days. However, no repeated exposure, whether at 2–3 or 7 day intervals, was as effective as the first NO2exposure in stimulating incorporation of [3H]thymidine.