Windup--the gradual increase of the response--of dorsal horn neurons to repeated activation of primary afferents is an elementary form of short-term plasticity that may mediate central sensitization to pain. In deep dorsal horn neurons of the turtle spinal cord in vitro we report windup of the response to repeated depolarizing current pulses as well as to repeated stimulation of the ipsilateral dorsal root. We found both forms of windup to be mediated by a depolarizing potential produced by increasing activation of postsynaptic L-type Ca2+ channels. These results suggest a central role for intrinsic postsynaptic properties in nociceptive plasticity and for L-type Ca2+ channels as a promising target for therapeutic intervention.