Action potentials in pyramidal neurons are typically followed by an afterdepolarization (ADP), which in many cells contributes to intrinsic burst firing. Despite the ubiquity of this common excitable property, the responsible ion channels have not been identified. Using current-clamp recordings in hippocampal slices, we find that the ADP in CA1 pyramidal neurons is mediated by an Ni2+-sensitive calcium tail current. Voltage-clamp experiments indicate that the Ni2+-sensitive current has a pharmacological and biophysical profile consistent with R-type calcium channels. These channels are available at the resting potential, are activated by the action potential, and remain open long enough to drive the ADP. Because the ADP correlates directly with burst firing in CA1 neurons, R-type calcium channels are crucial to this important cellular behavior, which is known to encode hippocampal place fields and enhance synaptic plasticity.