We compared the relative sensitivity of chicken embryos exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) injected either into the yolk or into the air cell. The TCDD was injected at the start of incubation (embryonic day 0) and the embryos were sacrificed at multiple times during embryonic development. A subset of embryos were allowed to hatch undisturbed. The chick embryo was significantly more sensitive to TCDD when injected into the yolk than when injected into the air cell. The resultant median lethal dose (LD50) (122 pg/g egg, determined by probit analysis; 146 pg/g egg determined by interpolation) was 60% lower than the LD50 (297 pg/g egg by probit; 255 pg/g egg determined by interpolation) for air-cell-injected TCDD. A significant decrease in hatch weight of embryos exposed to high concentrations of TCDD compared to controls occurred, and this decrease was even more pronounced at a lower concentration in the yolk-injected birds. Interestingly, during the period of embryonic days 11 through 15, the mean weight of the yolk-injected embryos was smaller than the mean weight of the air-cell-injected embryos. This difference was not noticably evident just before or just after this developmental period. Embryos exposed to high concentrations of TCDD injected into either the yolk or the air cell tended to die within the first 2 weeks of incubation. A number of TCDD-exposed embryos survived the entire 21-d incubation period, but only air-cell-injected embryos were able to hatch successfully. Because the injection site varies in studies reported by different laboratories, the relative sensitivity must be considered when comparing results from different studies.