Active and passive membrane properties of dorsal root ganglion (DRG)-cells from the intact superfused ganglion of pigeons have been compared with the conduction velocity of their centrifugal axons. About two thirds of the neurones were associated with myelinated axons and classified as A-cells; the remainder were associated with unmyelinated axons and classified as C-cells. Slowly conducting group III A-cells (5--25 m . s-1) constituted half of the A-cell population. With exception of spike duration, spike parameters and membrane properties did not differ among the A-cells. Spike duration increased with decreasing conduction velocity demonstrating a small plateau ("hump") during the fall time in group III neurones. This hump was more distinct in C-cells, resulting in a 2--5 times longer duration of action potentials. Amplitude and duration of afterhyperpolarization (AHP) of C-cells was 2--3 times that of A-cells. Administration of 10 mM CoCl2 decreased the rate of rise and the overshoot but increased the rate of fall of the action potential in C-cells and group III A-cells, largely abolishing the hump. It is suggested that the hump of the spike potential is largely produced by a Ca-current and that the resultant increase of intracellular Ca might produce the larger AHP in C-cells, secondary to an increase in K-conductance.