Spinal serotonin1 (5-HT1)(labelled by [3H]5-HT), 5-HT1A (labelled by [3H]8-hydroxy-2-(di-n-propylamino)tetralin ([3H]8-OH-DPAT)), mu- (labelled by [3H]Tyr-D-Ala-Gly-(Me)Phe-Gly-ol ([3H]DAGO) and [3H]naloxone) and delta-opiate (labelled by [3H]Tyr-D-Ser-Gly-Phe-Leu-Thr [( 3H]DSTLE] receptor binding sites were studied in adult rats using quantitative autoradiography after either neonatal treatment with capsaicin or unilateral cervical dorsal rhizotomy. Both treatments produced a significant loss of 5-HT (-20 to -30%) and opiate (-30 to -45%) binding sites within the superficial layers of the dorsal horn, suggesting they are partly located presynaptically on primary afferent fibres. Thus, 5-HT, as well as opiates, might generate analgesia by acting--at least partly--on primary afferent nociceptive fibres at the spinal level.