The surfactant system, a complex mixture of lipids and proteins, controls surface tension in the lung and is crucial for the first breath at birth, and thereafter. Heterokairy is defined as plasticity of a developmental process within an individual. Here, we provide experimental evidence for the concept of heterokairy, as hypoxia induces a change in the onset and rate of development of surfactant, probably via endogenous glucocorticoids, to produce individuals capable of surviving early hatching. Chicken eggs were incubated under normoxic (21% O₂) conditions throughout, or under hypoxic (17% O₂) conditions from day-10, of incubation. Embryos were sampled at days 16, 18, and 20 and also 24h after hatching. In a second experiment, dexamethasone (Dex), tri-iodothyronine (T₃) or a combination (Dex+T₃) were administered 24 and 48h before each time point. Both hypoxia and Dex accelerated maturation of the surfactant lipids by increasing total phospholipid (PL), disaturated phospholipid (DSP) and cholesterol (Chol) in lavage at days 16 and 18. Maturation of surfactant lipid composition was accelerated, with day-16 %DSP/PL, Chol/DSP and Chol/PL resembling the ratios of day-20 control animals. The effect of Dex+T₃ was similar to that of Dex alone. Hypoxia increased plasma corticosterone levels at day 16, while plasma T₃ levels were not affected. Hence, exposure to hypoxia during critical developmental windows accelerates surfactant maturation, probably by increasing cortiocosterone production. This internal modulation of the developmental response to an external stimulus is a demonstration of physiological heterokairy.