14C-Nicotine-HCl (4.5 mg) was injected into the air space of chick eggs after one day of incubation and the distribution of radioactivity measured on days 3, 5, 7 and 12. By day 3, 98% of the dose had passed from the air space into the egg, proper. The quantity of nicotine in the yolk was highest on day 3 (74% of dose) and decreased steadily until day 12 (52%). The quantity in the albumen increased from 16% on day 3 to a maximum on day 7 and remained constant at 34% until day 12. The embryo contained 0.03% of the nicotine on day 3, and this level increased to 5% by day 12. Nicotine in the allantois increased throughout the study period (0.01% on day 3 to 3% on day 12). Although only 5% of the dose was present in the embryo, the highest concentrations within these components of the embryo ranged from a minimum of 0.85 microng nicotine/mg dry wt in the heart to a maximum of 2.00 microng nicotine/mg dry wt in the carcass. These levels within the embryo were two to five times higher than the greatest concentrations in the yolk and albumen. The dynamics of nicotine passage from the air space into other components of the fertilized chick egg were clarified. Whereas the conventional expression of teratogenic doses of drugs in chick eggs (i.e., ppm of egg weight) assumes that the compound is distributed uniformly, this study indicates that such expressions are inaccurate.