The electrical characteristics of individual rat dorsal root ganglion neurones were studied and related to the peripheral axon conduction velocity and morphological cell type. Neurones were divided into four groups based on the conduction velocity of their peripheral axons (A alpha, 30-55 m/s; A beta, 14-30 m/s; A delta, 2.2-8 m/s and C less than 1.4 m/s). Electrophysiological parameters examined included membrane potential, action potential amplitude and duration, after-potential height and duration, input resistance and the occurrence of time-dependent rectification. The mean duration of the somatic action potentials was found to be characteristic for each of the conduction velocity groupings. However, there was considerable overlap between groups. The fast-conducting (A alpha) and slowly conducting (A delta) myelinated fibres had short-duration action potentials, within the ranges 0.49-1.35 and 0.5-1.7 ms at the base respectively. The A beta and C cells had somatic action potentials with durations in the ranges of 0.6-2.9 and 0.6-7.4 ms respectively. The longer action potential durations could be related to the presence of an inflexion on the repolarizing phase seen in a third of A beta neurones (called A beta I neurones) and in all C neurones. The action potential overshoot was larger in C neurones and A beta I neurones than in the other neurone groups. The mean duration of the after-hyperpolarization was several times greater in C neurones than in A neurones. A delta neurones displayed the shortest and greatest amplitude after-hyperpolarizations. Large, long-lasting after-hyperpolarizations were not limited to neurones displaying an inflexion. The electrophysiological properties of the soma membrane of A delta neurones closely resembled those of A alpha neurones, while in several respects those of C neurones resembled the A beta I neuronal properties. The input resistance was found to be much greater in C than in A cells, although there was no significant difference between specific membrane resistance values calculated for the different groups. A number of A cells exhibited time-dependent rectification.