Oleamide (cis-9,10-octadecenoamide) is an endogenous brain lipid which has been suggested to induce sleep in experimental animals. The mechanism of action is unclear but shares many of the characteristics of endogenous cannabinoids such as anandamide and has been shown to enhance in vitro responses to 5-HT and GABA. In the present study we investigated the effects of oleamide on two motor behaviours, back muscle contractions (BMC) and wet-dog shakes (WDS) induced in rats by treatment with the 5-HT2 receptor agonist DOI ((+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane hydrochloride). We then examined the potential involvement of CB1 cannabinoid receptors in the responses to oleamide and the mechanism of interaction between CB1 and 5-HT2 receptors. Oleamide and the cannabinoid receptor agonist HU210 (6aR)-trans-3-(1,1-dimethylheptyl)6a,7,10,10a-tetrahydro-1-h ydroxy-6,6-dimethyl-6H-dibenzo[b,d]pyran-9-methanol) produced a hypolocomotion which was prevented by the CB1 antagonist SR141716A (N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-me thyl-1H-pyrazole-3-carboxamide hydrochloride). Despite having no effect alone, oleamide and HU210 potentiated BMC induced by treatment with DOI. SR141716A alone did not affect the response to DOI but it blocked the potentiations caused by oleamide or HU210. WDS were unaffected by oleamide and slightly reduced by HU210. In vitro, oleamide and HU210 enhanced the high affinity binding of 5-HT to 5-HT2 receptors on rat cerebral cortex membranes labelled with 3H-ketanserin. Neither agent, however, altered 5-HT-stimulated phosphoinositide hydrolysis in rat cerebral cortex slices. Oleamide occupied CB1 cannabinoid receptors on rat brain membranes labelled with 3H-CP55940 with an IC50 of 10 microM. The data presented are consistent with oleamide acting via a cannabinoid recognition site to enhance 5-HT2 receptor function in vivo. The mechanism of the modulation is still unclear but it does not appear to involve a potentiation of 5-HT2 receptor-stimulated phosphoinositide hydrolysis.