In this study, broiler embryos were exposed during the second half of incubation [embryonic day (ED) 10 until ED18] to 4% CO2. The CO2 was set to reach 2% on ED11 and 4% from ED12 onward. Two experiments were conducted with the same setup. Embryo weight was measured and partial pressure Of CO2 and O-2 in the air cell was analyzed at several embryonic ages. Times of internal pipping, external pipping, and hatching were recorded. Chicks were raised until d 7 posthatch. Plasma corticosterone, triiodothyronine, and thyroxine concentrations were determined. Embryonic growth was not retarded and hatchability did not decrease in the CO2-incubated group, demonstrating that chicken embryos can tolerate high (4%) concentrations of CO2 between ED10 and ED18. In the first experiment, partial pressure Of CO2 in the air cell was significantly higher in the CO2 group on ED11, ED12, ED13, and ED14, but disappeared thereafter. This difference was not observed in the second experiment. A change in the hatching process of the CO,, group was seen. Relative growths of newly hatched chicks until d 7 posthatch were equal in the CO2 group and the control group. However, corticosterone and thyroxine concentrations were significantly higher in the CO2-incubated chicks on d 7 posthatch.