Recent studies show the importance of differential CO(2) levels during the first half of incubation of chicken eggs on embryonic and postnatal growth. However, it is not known how external higher CO(2) levels affect embryonic acid-base balance. In this study, the effect of an early rise in CO(2), between 25th and 96th hour of incubation to 1.5% and maintained at that level until 240h of incubation, was investigated on air cell gases, blood gas parameters from ED10 onwards and on embryonic growth and hatching parameters. Higher external CO(2) concentrations resulted in a faster acidification of albumen resulting in a faster decrease of albumen pH with development, illustrating the capacity of albumen to cope with higher environmental CO(2). Moreover, [Formula: see text] in blood was higher in CO(2) incubated embryos at embryonic day 10 and 11 but without a change in blood pH. The additional increase in plasma HCO(3)(-) concentration at day 10 and 11 was responsible for buffering the higher [Formula: see text] in CO(2) incubated embryos in order to stabilize pH. However, effects of hypercapnia on blood acid-base parameters extinguished 2 days after termination of high CO(2) incubation. Embryonic growth was modestly accelerated which was reflected in higher embryonic weights at day 6 and 10 and a significant earlier hatching; hatchling weights were not different between treatment groups.