We developed a high-performance liquid chromatography/mass spectrometry (HPLC/MS) method for the identification and quantification of anandamide, an endogenous cannabinoid substance, and other fatty acid ethanolamides (AEs) in biological samples. Using a mobile-phase system of methanol/water and gradient elution, we achieved satisfactory resolution of all major AEs, including anandamide, palmitylethanolamide (PEA), and oleylethanolamide (OEA). Electrospray-generated quasi-molecular species were used as diagnostic ions and detected by selected ion monitoring (SIM). Synthetic deuterium-labeled AEs were used as internal standards, and quantification was carried out by isotope dilution. A linear correlation (r 2 ؍ 0.99) was observed in the calibration curves for standard AEs over the range 0 -0.5 nmol. Detection limits between 0.1 and 0.3 pmol per sample and quantification limits between 0.5 and 1.2 pmol per sample were obtained. The method was applied to the quantification of anandamide, PEA, and OEA in plasma prepared from rat blood collected either by cardiac puncture or by decapitation. After cardiac puncture, AE levels were in the low-nanomolar range: anandamide, 3.1 ؎ 0.6 pmol/ml; PEA, 9.4 ؎ 1.6 pmol/ml; OEA, 9.2 ؎ 1.8 pmol/ml (mean ؎ SE, n ؍ 9). By contrast, after decapitation AEs were dramatically elevated (anandamide, 144 ؎ 13 pmol/ml; PEA, 255 ؎ 55 pmol/ml; OEA, 175 ؎ 48 pmol/ml). Thus, disruptive procedures of blood collection may result in gross overestimates in the concentrations of circulating AEs.