Intrathecal (ITH) administration of 5 micrograms 6-hydroxydopamine (6-OHDA) in mice selectively lesioned descending catecholaminergic pathways. Uptake of 3H-noradrenaline (3H-NA) into synaptosomes from the lumbar spinal cord was reduced by 95%, without any change in the uptake of 14C-5-hydroxytryptamine (14C-5-HT). Synaptosomal uptake of 3H-NA and 14C-5-HT in the brain was not altered. The nociceptive sensitivity was evaluated using the tail-flick, hot plate and formalin tests 3 and 14 days after injection of 6-OHDA. At day 3 hyperalgesia was found in the hot-plate test, unchanged response latency in the tail-flick test and hypoalgesia in the formalin test. At day 14 there were no statistically significant differences from controls in any of the tests. The present findings support the contention that catecholaminergic pathways participate in the tonic regulation of nociception in the spinal cord. However, while supraspinally integrated responses to acute thermal pain, as measured with the hot-plate test, are inhibited by these pathways, responses to prolonged chemical pain are enhanced.