This study found that in lightly-anesthetized rats a unilateral micro-injection of glutamate (200 mm, 0.5 µl) into the thalamic nucleus submedius (Sm) markedly depressed the radiant heat-evoked tail flick (TF) reflex. After injection, the mean TFL increased 25.6+/-6.5% (n=24) of the baseline at 5 min, up to a peak value (48.4+/-7.2%) at 20 min, and recovered to the baseline level at 60 min. This inhibitory effect was dose-related and repeatable over a time interval of 1.0-1.5 h in the same animal. Furthermore, micro-injections of gamma-aminobutyric acid (GABA) (100 mm) into the ipsilateral ventrolateral orbital cortex (VLO) (0.7 µl), or bilaterally into the lateral or ventrolateral parts of the periaqueductal gray (PAG) (0.5 µl on each side), eliminated the Sm-evoked inhibition. After GABA was injected into VLO or PAG, the Sm applications of glutamate failed to produce any significant changes in TFL, with the TFL changes being similar to the saline control (p>0.05). These results confirmed our previous findings that electrical stimulation of Sm depressed the rat TF reflex and that this inhibitory effect was blocked by electrolytic lesion of the VLO or PAG. Therefore, the present study provides further support for the hypothesis that Sm plays an important role in modulation of nociception, and that its effects are mediated by the VLO-PAG pathway, leading to activation of the brainstem descending inhibitory system and depression of the nociceptive inputs at the spinal cord level. Copyright 1998 European Federation of Chapters of the International Association for the Study of Pain.