The aim of this experiment was to investigate the role of the gamma-aminobutyric acid (GABA)-ergic transmission in the nociception within the spinal trigeminal nucleus. The formalin test was used as an animal model of inflammatory pain. Two groups of six rats were used. The behavioural response to the labial injection of formaldehyde (50 microl of a 5% solution) (group 1) or saline (group 2) was evaluated by recording the time spent in facial grooming during a period of 8 min (one period before and seven consecutive periods after the injection). The extracellular concentration of GABA in the trigeminal caudalis nucleus was evaluated, during the formalin test, on samples of 30 microl each (one sample before and three samples after the labial injection) obtained by microdialysis and analysed by HPLC with electrochemical detection of the o-phtalaldeyde pre-column derivate. Subsequently, three more groups of six rats each were injected with saline, muscimol (GABAa receptor agonist), or bicuculline (GABAa receptor antagonist) in the trigeminal caudalis nucleus, before performing the formalin test. The injection of formaldehyde induced a biphasic behavioural response and an increase of the GABA levels at 15-45 min. The injection of bicuculline, but not muscimol or saline, strongly decreased the behavioural response of the formalin test. These findings suggest that GABAergic neurons in the trigeminal caudalis nucleus are involved in the transmission of nociceptive information.