A study was carried out on the effect of topical application of 3.0 micrograms/microliters noradrenalin (NA) to the obex of awake guinea pigs bearing electrolyte lesions and submitted to noxious peripheral stimulation (electric pulses). The animals with extensive lesions covering the area postrema (AP) and the solitary tract nucleus (STN) showed a marked increase in vocalization (V) and defense motor response (M) to the noxious peripheral stimulus after NA application. In the group bearing lesions restricted to the AP, the analgesic effect of NA was potentiated, with a marked decrease in V and M after noxious peripheral stimulation. These results suggest that the integrity of the STN is a fundamental requirement for the analgesic effect of NA topically applied to the obex, and that the AP may have an inhibitory tonic action on the STN in the modulation of the noxious reaction.