We investigated the effects of sustained embryonic hypoxia on the neonatal ventilatory chemosensitivity. White Leghorn chicken eggs were incubated at 38°C either in 21% O₂ throughout incubation (Normoxia, Nx) or in 15% O₂ from embryonic day 5 (Hypoxia, Hx), hatching time included. Hx embryos hatched about 10 h later than Nx, with similar body weights. Measurements of gaseous metabolism (oxygen consumption, V_O2) and pulmonary ventilation (VE) were conducted either in the early or late hours of the first post-hatching day. In resting conditions, Hx had similar V_O2 and body temperature (Tb), and slightly higher VE and ventilatory equivalent (VE/V_O2), than Nx. Ventilatory chemosensitivity was evaluated from the degree of hyperpnea (increase in VE) and of hyperventilation (increase in VE/V_O2) during acute hypoxia (15 and 10% O₂, 20 min each) and acute hypercapnia (2 and 4% CO₂, 20 min each). The chemosensitivity differed between the early and late hours, and at either time the responses to hypoxia and hypercapnia were less in Hx than in Nx, because of a lower increase in VE and a lower hypoxic hypometabolism. In a second group of Nx and Hx hatchlings the VE response to 10% O₂ was tested in the same hatchlings at the early and late hours. The results confirmed the lower hypoxic chemosensitivity of Hx. We conclude that hypoxic incubation affected the development of respiratory control, resulting in a blunted ventilatory chemosensitivity.