Activation of the cardiopulmonary reflex arc by volume expansion results in antinociception of the tail-flick response to radiant heat, and this antinociception can be attenuated by resection of the right vagal nerve trunk. Resection of the right vagal nerve trunk also attenuates foot-shock induced analgesia ( FSIA ), but this vagal influence appears to be qualitatively different from that produced by volume expansion. These findings indicate that pain perception is modulated by cardiovascular input.