Infant rats, together with their mother, were exposed to the simulated altitude of 7,000 m for 8 hours per day since birth to the age of 17 days. Animals were studied the 25th day, 8 days after the last exposure to hypoxia. The experimental and control animals were sacrificed the 25th day by the transaortic perfusion with 4% buffered neutral formaldehyde under ether anaesthesia. Brains were processed for classical neurohistological analysis (Nissl staining), Fluoro-Jade B and Hoechst. Cortical area in the AP plane 3 mm posterior to bregma was subjected to quantification and "laminar analysis" of the neurones count. The findings were as follows: a) The cytoarchitectonics of the brain in animals exposed to hypoxia was not severely damaged. b) The thickness of neocortex is in the experimental animals lower than that in the controls. c) The "laminar analysis" of neocortex showed a relative increase of neuronal density in layers I., II., V. and VI. of the cortex. d) The electrical stimulation of sensorimotor cortex 8 days after the end of hypoxia brought about prolongation of evoked cortical after discharges. These results demonstrate that the intermittent hypobaric hypoxia has a profound effect on morphological maturation of the central nervous system in infant rats. Hypoxia influenced the excitation-- inhibition mechanisms of cortical neurones.