The prenatal treatment of timed pregnant mice was conducted as described in legend to Figure 2, and fetuses (− 1 PND), and newborn mice were sacrificed at birth (0 PND), 1, 3, and 5 PND. Lungs were excised and microsomes were prepared in individual mice, and EROD activities were measured as described under Materials and Methods. Values represent means ± SE of at least 5 mice from each group. *, Statistically significant differences between each group versus the CO + air-breathing group at each time point at P < 0.05. CYP1A1 apoprotein expression in individual microsomes was determined by Western blotting, and the figure shows a representative Western blot.