Layers I and II of the somatic sensory cortex are clearly distinguishable with the electron microscope because of characteristic differences in the number, type and orientation of neurons and dendritic and axonal ramifications. Layer I may be subdivided into: (i) a subpial astrocytic layer immediately deep to the basement membrane of the cerebral surface; (ii) a superficial quarter consisting of bundles of small myelinated axons and large numbers of small axon terminals which contain spherical vesicles and end in asymmetrical synaptic complexes mainly on large dendritic spines. Most of these terminals are derived from a dense feltwork of fine unmyelinated axons which are especially concentrated at the junction of the superficial and deep parts of layer I; (iii) a deeper three quarters with similar features to the above but with the additional characteristic of many obliquely orientated large dendrites which are the diverging branches of apical dendrites ascending from deeper layers. Small pyramidal neurons dominate layer II, but among them are a small number of non-pyramidal neurons whose beaded dendrites are covered with axon terminals. Large apical dendrites traverse this layer, and in addition to the typical asymmetrical synapse on dendritic spines, a few symmetrical types appear. These are derived from thin unmyelinated axons orientated horizontally within the layer, and the terminals contain many small flattened or pleomorphic synaptic vesicles.