ABSTRACT Exposure to increasing concentrations of CO2 in the first 10 days of incubation may have effects on the development of bird cardiac and respiratory organs. Moreover, the age of breeders can influence hatching performances. This study aimed to investigate the effect of exposure to increasing concentrations of CO2 in the first 10 days of incubation on the morphophysiological development of the digestive system of embryos and chicks from broiler breeders aged 31 and 41 weeks. A total of 860 fertile eggs from the Cobb strain were distributed in a completely randomized design, in a 2 x 2 factorial arrangement, with 2 different gaseous environments (Control (C) - no increase in CO2 concentration and, Hypercapnia (CO2) - a gradual increase in CO2 concentration until reaching 1% on the 10th day) and 2 different broiler breeder ages (31 and 41 weeks). Half of the eggs were obtained from 31-week-old breeders, and the other half from 41-week-old breeders. Compared to the control group, incubation in an atmosphere with 1% CO2 led to an increase in villus heights in the duodenum, jejunum, and ileum of the embryos, with a reduction in villus density in the same segments. Chicks from 41-week-old broiler breeders showed higher villus heights in the duodenum, jejunum, and ileum at day 1 post-hatching, and lower villus density at 7 days. It was concluded that the incubation of fertile eggs in hypercapnia conditions could positively affect the small intestine of embryos and post-hatch chicks.