Bruxism, principally jaw clenching, is frequently observed in users of the recreational drug 3,4-methylenedioxymethamphetamine (MDMA). It has been suggested that during bruxism a reduction of the activity of oral protective reflexes occurs. In this study we investigated the effects of intravenously administered MDMA on the digastric electromyographic responses elicited by orofacial electrical stimulation in the rat. We also assessed the effects of either the administration of a single dose (20 mg kg(-1), s.c.) or repeated doses of MDMA (same dose, twice a day, for 4 d) on the jaw-opening reflex (JOR) and on the sensitivity of the alpha(2)-adrenoceptors which, in an inhibitory way, regulate it. Increasing doses of MDMA (1-29440 micro g kg(-1)) induced an incomplete inhibition of JOR and 50% inhibition (ED(50)) at 2550 micro g kg(-1); maximal inhibition was 88%. The repeated treatment with MDMA led to an enhancement of the inhibition of JOR induced by the alpha(2)-agonist, clonidine (ED(50) was reduced by 77%), indicating an increased sensitivity of the alpha(2)-adrenoceptors. This study shows that the intravenous administration of MDMA reduces the JOR while repeated doses of the drug enhance the inhibitory noradrenergic mechanisms which regulate the reflex. The results also allow speculation that a reduction of JOR may underlie the occurrence of episodes of bruxism in MDMA users.