The behavioural response to intraplantar injection of inflammatory mediators was examined using the rating scale developed to assess formalin-induced pain. Serotonin, bradykinin, prostaglandin E2, substance P and histamine induced dose-dependent favouring of the injected paw. Serotonin, bradykinin and prostaglandin E2 also induced transient dose-dependent paw elevation (lifting) and licking. Noradrenaline produced only a weak favouring response. Serotonin produced a synergistic increase in lifting and licking when combined with any of the other mediators, while all other combinations of agents taken two at a time showed additivity. There was apparent antagonism between some combinations in the favouring response; this may reflect overestimation of the baseline. The data indicate that (i) the overt spontaneous behaviour of rats can be used to evaluate spontaneous pain, (ii) the favouring response and the lifting and licking responses are qualitatively different, the former being similar to hyperalgesia and the latter possibility representing overt pain, (iii) hyperalgesia and overt pain are related, but the generation of overt pain involves specific mechanisms in addition to those required to induce hyperalgesia, and (iv) serotonin may function to enhance the pain-producing effects of inflammatory mediators, even when they lack intrinsic activity. The data show that some inflammatory mediators produce transient overt pain when high doses are injected into normal tissue in rats. Combination of inflammatory mediators with low doses of serotonin produced a synergistic increase in the pain response. The data suggest that serotonin released from platelets in injured tissue plays a central role in the pain associated with injury, and that serotonin antagonists may have promise as peripherally acting analgesics or analgesic adjuncts by blocking a synergistic process involved in algogenesis.