The analgesia produced by tricyclic antidepressants (TCAs) in rats and the role of serotonin in this analgesia have been investigated in this study. Rats received intraperitoneal injections of imipramine and amitriptyline, which are TCAs, and the serotonin synthesis inhibitor, pCPA. An acute analgesic effect was measured 90 min after the first injections; a chronic effect was measured 24 h after the last injections, on the 7th and 15th days using the hot-plate method. Both antidepressants elevated the pain threshold acutely, while pretreatment with pCPA largely blocked the analgesia. Based on these data it was found that in the acute case both of the antidepressants potentiate an endogenous analgesia mechanism, which acts on the serotonergic system. After long-term use, amitriptyline, which acts on serotonin, had an analgesic effect and this effect was blocked by pCPA. Imipramine, which acts on noradrenaline, had no effect on the pain threshold in chronic use. Consequently, serotonin is an important link in TCA analgesia; noradrenaline has no effect.