Interactions of anandamide (N-arachidonylethanolamide), an endogenous compound for cannabinoid receptors, with the receptors for 5-hydroxytryptamine (5-HT), benzodiazepine, and γ-aminobutyric acid_A (GABA_A) receptors in bovine synaptic membrane were examined. Anandamide decreased the 5-HT receptor bindings at concentrations of 1-100 μM, although it did not cause any change in benzodiazepine or GABA_A receptor bindings. A high concentration of anandamide, 100μM, significantly decrease both [³H]5-HT and [³H]ketanserin bindings. The present study revealed that the pharmacological activity of anandamide might be partially mediated through the 5-HT receptor.