It is known that adult neural stem cells are found in a hypoxic environment and are sensitive to oxygen tension. However, whether oxygen tension also regulates neural progenitor behaviour during development has been unclear. Now (p. 2904), Alexander Storch and co-workers investigate the effects of maternal hypoxia or hyperoxia on cortical development in mice. They find that exposure of pregnant mothers to different atmospheric oxygen tensions causes changes in foetal brain oxygenation, and that this has consequences on overall brain volume: hypoxic embryos have smaller brains, whereas hyperoxia causes increased brain size. At the cellular level, the hypoxic foetal cortex shows reduced proliferation of progenitors and increased apoptosis. By contrast, hyperoxia induces the accumulation of a population of proliferative progenitor cells in regions of the cortex more basal to the main zones of proliferation. These cells, similar to the outer subventricular zone progenitors that are normally rare in rodents but more common in primates, apparently contribute to increased corticogenesis. Although the mechanisms by which oxygen signalling regulates cortical development remain unknown, these intriguing results point to an important role for oxygen tension during foetal neurogenesis.