Combining intracellular recording and dye-injection techniques permitted direct correlation of neuronal soma size with peripheral nerve conduction velocity in individual neurones of the L4 dorsal root ganglion (d.r.g.) of the anaesthetized 5-8-week-old rat. The conduction velocities fell into two main groups; those greater than 14 m/s (A alpha and beta fibres) and those less than 8 m/s (A delta and C fibres). Fibres with conduction velocities in the A delta range (2.2-8 m/s) in the sciatic nerve between the sciatic notch and the neuronal soma in the d.r.g. often conducted more slowly, that is in the C-fibre range (less than 1.4 m/s), in the periphery from the tibial nerve to the sciatic notch. For the fast-conducting myelinated afferents, there was a loose positive correlation between cell size and the conduction velocity of the peripheral axon, whereas a clearer positive correlation existed between neuronal cell size and axonal conduction velocity both for A delta- and for C-fibre afferents. The relationship of the cell cross-sectional area (measured at the nucleolar level), to the cell volume for each neuronal soma was similar for the different conduction velocity groups. The somata of the fast-conducting myelinated A alpha and A beta fibres had a similar mean and range of cross-sectional areas to those of the large light cell population. The somata with A delta and C fibres were of a more uniform size and were restricted to the smaller cells within the ganglia. The mean and range of cross-sectional areas of the C cells was similar to those of the small dark cell population. A delta somata had a larger mean and range of cell sizes than those of the small dark cell population. The relationships of peripheral axon type to the morphological cell types are discussed.