In 2 cats nerves of the spinal pia mater were studied by means of electron microscopy. These nerves possessed no perineurium. However, it has been observed that flat cells of the surrounding connective tissue can form an incomplete covering for small nerve bundles. In addition the perivascular nervous plexus of subarachnoid arteries of the brain stem, which were examined in 1 animal, also showed no perineural sheath. These results are discussed with those reported in the literature dealing with the perineurium of other peripheral nerves. The observations concerning the distribution and the composition of leptomeningeal nerves are in accordance with those obtained by light microscopic investigations.