After perfusion fixation with a mixture of paraformaldehyde, glutaraldehyde, and potassium-chromate 4 adult human medullary pyramids were examined with the electron microscope. The myelin sheaths showed only mild signs of post mortal destructions. The periodicity of the lamellae is preserved and varies about 17 nm. For quantitative purposes specimens were taken at random from regions with an extend of half a mm2 on the cross section of the pyramid. On the average 62 lamellae (s = 5, n = 200) built up 1 micron myelin sheath in the 4 brains. The myelin sheath thickness and the inner circumference of the myelin sheath are measured. The axon diameter is calculated from the inner circumference. We obtained an average number of 66,000 fibres, s = 11,000 per mm2. Fibres with a total diameter below 4 microns amount to 87.9%, fibres from 4 to 10 microns to 10.77%, and larger than 10 microns to 1.4%. An axon diameter of about 0.5 micron combined with a thickness of the myelin sheath of 0.3 micron was most often found in all 4 cases. The fibres showed a broad variation of myelin thickness. In a given group of the small axons the thickest myelin sheath exceeded 3 to 4 times the value of the thinnest one. The coefficient of correlation between axon diameter and myelin sheath thickness was calculated as r2 = 0.56. No correspondence exists between frequency maxima and the ratio g, axon diameter to total diameter. Below a total fibre diameter of 5 microns the ratio g scatters broadly about 0.6, fibres larger than 5 microns exceed the value of 0.6. From the morphological data only large sized fibres can evoke a strong effect on compound action potentials if the method of reconstruction of the action potential from fibre size frequency distribution is employed.