Numerous anatomical, pharmacological and electrophysiological data described in the literature indicate that spinal enkephalinergic and serotoninergic systems are probably involved in the control of nociceptive inputs from the periphery to the cerebral cortex. However, reported evidence was generally indirect and did not provide a real demonstration of the physiological participation of these neurones in pain control. This led us to select appropriate experimental approaches for studying directly the activity of spinal enkephalinergic and serotoninergic systems in animals (rat, cat) submitted to noxious stimuli. Owing to two catheters introduced into the subarachnoidal space of anesthetized rats, it was possible to perfuse the whole spinal cord with an artificial cerebro-spinal fluid and thus collect the neuroactive compounds released by spinal neurones (at least those in superficial layers) under various experimental conditions. Using this technique, we observed that some (but not all) nociceptive stimuli such as intense pinching of the muzzle, intraperitoneal injection of acetic acid or noxious heat applied to the muzzle or the tail induced a significant increase in met-enkephalin release from the spinal cord (see fig. 2). Similar effects were observed following the blockade of enkephalin catabolism by thiorphan and bestatin (see fig. 1) indicating that they were not due to some alteration of peptidase activities but really involved the activation of spinal enkephalinergic systems. Since cervical cord transection suppressed the stimulatory action of noxious stimuli on spinal met-enkephalin release, it could be proposed that the mechanisms involved were not limited to the cord but depended on supraspinal structures. Bulbo-mesencephalic serotoninergic neurones projecting to the spinal cord might well correspond to such structures (or at least to some of them) since nociceptive stimuli (such as noxious heat applied to the tail) also evoked a marked increase of serotonin (5-HT) release at the spinal level (fig. 3). Such observations together with indirect evidence reported in the literature suggested therefore that the activation of spinal enkephalinergic systems triggered by noxious stimuli might result from excitatory influence due to descending serotoninergic projections. However, in vitro studies using slices of the dorsal zone of the rat lumbar cord did not reveal any stimulatory effect of 5-HT on the spontaneous or K+-evoked release of met-enkephalin (fig. 4).(ABSTRACT TRUNCATED AT 400 WORDS)