Several long chain N-acylethanolamines, including the proposed endogenous ligands of cannabinoid receptors, anandamide (N-arachidonoylethanolamine, C20:4 NAE) and N-palmitoylethanolamine (C16:0 NAE), as well as some of their putative biosynthetic precursors, the N-acyl-phosphatidylethanolamines, were found in lipid extracts of five species of bivalve molluscs, including Mytilus galloprovincialis, commonly used as sea food. The amounts of these metabolites, the most abundant being C16:0 NAE and N-stearoylethanolamine, appeared to increase considerably when mussels were extracted 24h post-mortem, but were not significantly affected by boiling the tissue prior to extraction. In particulate fractions of homogenates from Mytilus, where the existence of a highly selective cannabinoid receptor with an immunomodulatory function has been previously described, an enzymatic activity capable of catalyzing the hydrolysis of C20:4 NAE amide bond, and displaying similar pH dependency and inhibitor sensitivity profiles as the recently characterized 'fatty acid amide hydrolase' was found. The enzyme Km and Vmax for C20:4 NAE were 29.6 microM and 73 pmol/mg protein/min, respectively. These findings support the hypothesis that C20:4 NAE, never reported before in the phylum Mollusca, may be a mollusc physiological mediator, and suggest that edible bivalves may be a dietary, albeit limited, source of C16:0 NAE, whose anti-inflammatory properties, when administered orally in amounts higher than those reported here, have been previously reported.