Glycine is a candidate nociception inhibitory transmitter in specific brain regions, like for example the spinal cord, the thalamic nuclei and the periaqueductal gray matter. However, quantitative changes in glycine released in these brain regions during peripheral inflammation episodes have not been characterized in awake animals. To address this issue, an in vivo microdialysis study was carried out in freely moving rats in order to analyse periaqueductal gray matter extracellular glycine concentration following unilateral formalin injection into the dorsal skin of the right hind-paw. The extracellular concentration of glutamine was also evaluated in order to analyse whether or not a non-neurotransmitter amino acid was equally modified. Intra-periaqueductal gray matter tetrodotoxin perfusion reduced extracellular glycine concentration (-44+/-5%), but did not change the glutamine dialysate values. Peripheral injection of formalin reduced the glycine release during the early phase (-62+/-8%) and the late phase (-36+/-6%) of hyperalgesia, although not during the analgesic period. Perfusion with naloxone (300microM) neither prevented the formalin-induced decreases in extacellular glycine concentration, nor modified the perfusate basal values of glycine and glutamine. These results show that, contrary to what has been recognized on the interactive role of opioids and GABA into the periaqueductal gray matter (i.e. opioid disinhibition), endogenous opioids seem not to modulate the activity of glycinergic neurons in the same midbrain area. In the light of these preliminary data, it is reasonable to suppose that GABA and glycine are probably not co-released at the level of periaqueductal gray matter of the rat.