The effects of microinjections of carbachol (Carb), serotonin (5-HT) and noradrenaline (NOR) into the spinal trigeminal nucleus (STN) of guinea pigs were studied in superficially anesthetized animals (nembutal, 40 mg/kg) submitted to noxious dental stimulation (electric shock). The defense motor response (DMR) was studied during noxious stimulation through the control period and up to 60 minutes after chemical stimulation. The agents were microinjected into four sites in the bulbar-spinal projection of the STN. Carb (1 microgram/microliter) caused significantly decreased DMR when injected into all regions studied, with total DMR abolition into the site located 1.0 mm caudal to the obex. 5-HT (30 micrograms/microliters) and NOR (30 micrograms/microliters) elicited a less intense effect than Carb, causing significant reduction in DMR only when injected into two sites. These results suggest cholinergic, serotonergic and adrenergic involvement of the mechanism of aversive response to dental pain.