Rat dorsal root ganglion neurons can be classified into at least three distinct groups based on cell size, afferent fiber diameter, electrophysiological properties, sensitivity to vanilloid agonists such as capsaicin, and function. In the present study, ATP-activated current in these neurons was characterized using whole-cell patch-clamp recording. Small diameter (<30 microm) cells had high capsaicin sensitivity, high affinity for ATP, and rapidly desensitizing ATP-activated current. Medium diameter (30-50 microm) cells had no capsaicin sensitivity, lower affinity for ATP and slowly desensitizing ATP-activated current. Large diameter (>50 microm) cells were insensitive to both capsaicin and ATP. These findings suggest that distinct types of ATP receptor-ion channels are expressed in different types of dorsal root ganglion neurons, and may contribute to the functional differences among these types of neurons.