Tricyclic antidepressants have been shown to be useful for the treatment of pain of varying etiology. Monoaminergic systems seem to be implicated in this phenomenon. In this study, the influence of the selective beta 1- (CGP 20712A) and beta 2- (ICI 118551) adrenergic blockers on the antinociceptive effect of desipramine and nortriptyline was studied in mice using physical and chemical nociceptive tests that implicate different levels of sensory-motor integration in the central nervous system (CNS). An activity test was performed to detect "false positive" or "false negative" results. Results obtained show that both CGP 20712A and ICI 118551 are able to antagonize the antinociceptive effect of these antidepressants in physical tests (hot-plate and tail-flick). However, in chemical tests (acetic acid and formalin), the analgesic effect of the antidepressants used was only antagonized by CGP 20712A. These results suggest that the analgesic effect of desipramine and nortriptyline is mediated by beta-adrenoceptors. The beta-adrenoceptor involved depends on the type of nociceptive stimulusbeta 1 and beta 2 are both implicated when the stimulus is physical, but only beta 1 is involved when the stimulus is chemical.