In a variety of mammalian neurons, a brief depolarization generates an afterdepolarizing potential that triggers the firing of a short series or burst of action potentials. Although such burst firing is thought to contribute to the processing of neural information, the ionic currents that underlie this phenomenon have not been established. In whole-cell patch-clamp experiments on dorsal root ganglion neurons, we have found that the current that underlies this type of burst firing is a transient low-threshold (T-type) Ca2+ current. The data suggest that the T-type Ca2+ current may play an important role in the processing of information in the nervous system by virtue of its ability to elicit burst firing in neurons.