This communication describes the application of the barometric technique to the measurements of the breathing pattern (tidal volume and breathing rate) and pulmonary ventilation (VE) in chicken embryos and hatchlings. The chamber-plethysmograph was separated into two sections, an animal compartment, maintained at incubation temperature, and a recording compartment kept at a lower temperature. In the case of the embryos, the eggshell itself represented the animal compartment. The hatchlings were accommodated into a "nest" compartment. A flow-through system permitted simultaneous measurements of oxygen consumption (VO2) and carbon dioxide production. Values of breathing rate corresponded to those counted visually in hatchlings while resting in the incubator, and data of resting VE were similar to those obtained by airflow plethysmography, a more invasive technique applicable only to the hatchlings. At rest, the ventilatory equivalent (VE/VO2) of the hatchlings was similar to that reported for adult birds, while in embryos it was much lower. Hypoxia (15 and 10% O2) and hypercapnia (2 and 4% CO2) caused hyperventilation (increase in VE/VO2), both in the hatchlings and in the embryos, these latter using hypometabolism as the main approach to hyperventilate in hypoxia. We conclude that the barometric technique can be adapted to the study of breathing and VE responses in avian embryos and hatchlings.