With a quantitative method and serial sections a study was carried out under the electron microscope of the perikaryal projections of the neurons in the thoracic spinal ganglia of cat and rabbit. These projections usually appear as finger-shaped evaginations which run roughly parallel to the surface of the nerve cell body. Their length ranges between 0.3 and 3.25 microns, and they show a nearly circular cross section with a rather uniform transverse diameter having an average value of about 0.2 microns. Both in cat and rabbit a very high correlation was found between the surface area of perikaryal projections and both the volume and smoothed surface area of the corresponding nerve cell body. Perikaryal projections increase the surface area of the nerve cell body by 43% in cat and 39.5% in rabbit. These findings support the idea that perikaryal projections in sensory ganglion neurons are normal formations, which maintain the surface-to-volume ratio above the critical level for metabolic exchanges.