Alterations in nociceptive behaviors evoked by stimulation of the nucleus gigantocellularis (NGC) were observed following lesions of the dorsal central gray (DCG) or parafascicularis nucleus (PF) in rats. Lesions of the DCG decreased affective responses and facilitated extinction of avoidance responding associated with NGC stimulation. Lesions of the PF disrupted escape and avoidance responding to NGC stimulation, although postlesion training improved avoidance but not escape performance. The NGC, without its rostral connections to the DCG or PF, does not generate sufficient information for either aversive affective responses or locomotor escape responses that are associated with stimulation of this structure. In contrast, the elicited motor reactions do not depend upon these rostral structures. This supports a role for NGC neurons in functions related to motor, rather than to affective or complex behavioral components of nociception. The differential disruption of the more complex behaviors by DCG and PF lesions provides behavioral evidence for functional differentiation of a nociceptive system involving the relay of information from the NGC to these rostral areas.