Conduction velocities of rat L4, L5 and L6 dorsal root ganglion neurons were measured in vitro, from several points on the peripheral nerve and dorsal root. Conduction velocities calculated from a single stimulation point (12-26 mm from the ganglion) proved accurate for fibres conducting up to 17 m/s in the peripheral nerve and up to 14 m/s in the dorsal root, but tended to underestimate the value for faster fibres. C-fibre neurons of the L4 and L5 ganglia had a unimodal distribution of conduction velocities below 1.3 m/s.These were discontinuous with A-fibre conduction velocities, which also had a unimodal distribution and had no clear Aδ peak. In contrast, conduction velocities of L6 ganglion neurons showed no discontinuity between C- and A-fibres, but had a clear Aδ peak.In A-fibre neurons, dorsal root conduction velocities were on average about 14% slower than, and linearly related to, those in the peripheral nerve. However, in individual neurons the dorsal root conduction velocity could be slower or faster than that in the peripheral nerve. In C-fibre neurons dorsal root conduction velocities were almost always slower (average 28%) but not correlated with those of the peripheral nerve. Slowing of conduction velocity along the sciatic nerve was seen in most fibres conducting at less than 2 m/s, but not in faster fibres. The slowing was substantial (up to 60%), sometimes from the Aδ to the C-fibre range, and sudden, occurring at a distance of between 15 and 29mm from the ganglion.