Few studies have investigated the chronic cytokinetic effects of carcinogen exposure in the mouse skin. We report two experiments involving the repeated application of benzo[a]pyrene (BaP) to the dorsal skin of female Ha/ICR mice. In the first experiment, the cytokinetic, inflammatory, and DNA adduct responses were studied daily over a 9-day period encompassing the fourth and fifth weekly applications of BaP at doses of 16, 32, and 64 micrograms. The second experiment involved the same cytokinetic measurements at 1, 3, 5, and 8 months, and the weekly BaP doses were 4, 8, and 16 micrograms. The first study showed that after each application of 32 or 64 micrograms BaP, there was a wave of slow DNA synthesis in the epidermis which peaked at 24 hr, in coincidence with a wave of BaP-DNA adducts, followed by the appearance of dead and damaged keratinocytes. For the first few days after BaP application there was a depression in the mitotic rate which recovered several days before the next BaP application. There was a predominantly monocytic dermal inflammation throughout the observation period. In the second experiment, at the lower BaP doses, there was proliferative depression at 1 month, without dermal inflammation. With continued exposure, the proliferative depression changed to a dose-dependent increase in the rate of proliferation and dermal inflammation. The level of BaP-DNA adducts was followed in the 4 micrograms/week dose group, which showed a threefold increase after 4 months with the appearance of inflammation and heightened cell proliferation. These results suggest that the delayed inflammatory reaction, possibly based on a cell-mediated immune reaction to BaP, might have been responsible for the late cytokinetic responses and the associated increase in the level of BaP-DNA adducts.