In the awake restrained rat the intrathecal administration of substance P or the partial substance P homologue eledoisin-related peptide (ERP) reduced reaction time to a noxious radiant heat stimulus and, at high doses, produced additional behavioural responses suggesting that the animals had reacted to what they perceived as a painful stimulus. The reduction in tail-flick latency was observed as early as 30 sec following peptide administration peaked at 1 min and persisted for 5-10 min, after which an overshoot of the response (i.e., an increase in reaction time) was observed. The responses varied in their magnitude with the amount of peptide given, substance P being approximately 4 times more potent on a molar basis than ERP. Intrathecal administration of an equal volume of vehicle (artificial cerebrospinal fluid) had no effect on tail-flick latency and failed to produce any of the other behavioural changes. The following interpretations are made. The decrease in tail-flick latency suggests that pain threshold was decreased, and the dramatic behavioural effects seen at high doses suggest that an excess of substance P in the spinal cord is capable of producing a painful sensation. The rapid onset of the response suggests rapid penetration of substance P and ERP to the appropriate receptors, and the rapid decay of the response suggests rapid removal. Taken together, these results are consistent with the earlier suggestion that substance P plays a role as an excitatory agent in sensory pathways subserving pain. It is proposed that some conditions of chronic pain in man may therefore be due to an overabundant amount of substance P. This is complementary to a second proposal that other cases of chronic pain may be due to a supersensitivity of substance P receptors. The former is more likely to be associated with organic disorders, the latter with nerve damage, e.g. with causalgia, the neuralgias and perhaps some cases of phantom limb pain.