A case of a 5½-year-old patient with psycho-motor retardation is presented. It is studied by a cortico-subcortical biopsy of the right frontal lobe, by light and electron microscopy. Paraffin embedded material shows no alterations, but distinct presynaptic and axonal swellings are seen in plastic embedded material. Three morphological types of swelling are described, with increasing dimensions and decreasing number. We have suggested a metabolic disturbance which results in a storage of vesiculo-tubular-structures (which might be malformations of synaptic vesicles), with a chronological evolution of the axonic swelling, going from type 1 to type 3.