1. The functional consequences of P2X receptor activation on peripheral sensory neurones have been investigated in vivo. Behavioural indices of acute nociception were monitored in the conscious rat following subplantar injection of adenosine 5'-triphosphate (ATP), alpha,beta-methylene ATP, adenosine 5'-diphosphate (ADP) and adenosine. 2. Signs of overt nociception, i.e. hindpaw lifting and licking, were apparent in animals injected subplantar with the P2X receptor agonist, alpha,beta-methylene ATP. Nociceptive behaviours continued for 15 min following administration of alpha,beta-methylene ATP (200 nmol) and were dose-related (0-5 min hindpaw lifting times after injection of alpha,beta-methylene ATP 100 nmol and 1000 nmol were 89 +/- 26 s and 232 +/- 11 s, respectively). Subplantar ATP evoked a modest response only at the highest dose tested (1000 nmol; 0-5 min hindpaw lifting time 66 +/- 19 s) whilst ADP or adenosine (both 600 nmol) elicited negligible spontaneous nociceptive activity. 3. Morphine (3 mg kg-1, i.v.) abolished hindpaw licking behaviour induced by subplantar injection of either alpha,beta-methylene ATP (600 nmol) or bradykinin (1 nmol) and substantially reduced (88 +/- 5%) paw licking in formalin (0.5%, 0.1 ml) injected animals. In contrast, hindpaw lifting was only modestly inhibited (34 +/- 11%) in morphine-pretreated animals that had received subplantar bradykinin and was unaffected in rats in which the noxious stimulus was either subplantar alpha,beta-methylene ATP or formalin. Pretreatment of hindpaws with subplantar bupivacaine (1% w/v, 0.1 ml) abolished alpha,beta-methylene ATP-evoked nociceptive behaviours. 4. Hindpaw lifting and licking mediated by alpha,beta-methylene ATP (600 nmol, subplantar) were inhibited (72 +/- 15% and 95 +/- 5%, respectively) by 30 min local pretreatment with 600 nmol alpha,beta-methylene ATP. Subplantar alpha,beta-methylene ATP pretreatment did not inhibit behaviour stimulated by subsequent bradykinin (1 nmol) or formalin (0.5%, 0.1 ml) injection into the hindpaw. 5. Desensitization of small diameter sensory neurones with a single subplantar injection of capsaicin (100 micrograms) abolished all behaviours indicative of spontaneous nociceptive sensation in animals subsequently injected with alpha,beta-methylene ATP (600 nmol), bradykinin (1 nmol) or formalin (0.5%, 0.1 ml). 6. We conclude that activation of P2X receptors present on small diameter (capsaicin-sensitive) primary afferent neurones in the rat hindpaw mediates behaviour indicative of acute nociception.