Structural parameters of neuronal circuits were studied in rats exposed to aerogenic hypoxia during the postnatal development. In 18-day-old experimental animals pyramidal cells of the hippocampal CA1 area had less branches of the apical dendrites. These branches were shorter with lower dendritic spine density. Myelinated axons in the corpus callosum were less frequent. In electron-microscopic picture the mean section area of myelinated axons was lower, the myelin sheets were thinner and consisted of less myelin layers. Synapses in the II. layer of the somatosensoric cortex had more often flat pre-synaptic dense projections, smaller width between pre- and post-synaptic densities and thinner post-synaptic densities.