ceramides and sphingolipids ( 1, 2 ). Primary fatty acid amides (PFAM), R-CO-NH 2 with R being a long-chain fatty acid, were fi rst identifi ed from a biological source in 1989 with the identifi cation of palmitamide, palmitoleamide, oleamide, elaidamide, and linoleamide in luteal phase plasma ( 3 ). At the time of their discovery, the biological function of these mammalian PFAMs was unknown. Interest in the PFAMs dramatically intensifi ed with the discoveries that oleamide accumulated in the cerebrospinal fl uid (CSF) of sleep-deprived cats, that it is a natural component of the CSF in the cat, rat, and human, and that the administration of synthetic oleamide induced physiological sleep in rats ( 4 ). Intriguingly, later studies found that oleamide levels in the brain of the ground squirrel were ‫ف‬ 2.5-fold higher in hibernating animals relative to those found in nonhibernating animals ( 5 ). Other functions ascribed to oleamide, since its discovery as a sleep-inducing PFAM, include the ability to modulate gap junction communication in glial cells ( 6, 7 ), tracheal epithelial cells ( 8 ), seminiferous tubule cells ( 9 ), and fi broblasts ( 10 ); to allosterically activate the GABA A receptors and specifi c subtypes of the serotonin receptor ( 11-13 ); to affect memory processes ( 14 ); to increase food intake ( 15 ); to reduce anxiety and pain ( 16,17 ); to depress body temperature and locomotor activity ( 17,18 ); to stimulate Ca(II) release ( 19 ); and to relax blood vessels ( 20,21 ).Although much of the research regarding the PFAMs has focused on oleamide, studies show that some of the other known PFAMs are bioactive. Palmitamide is weakly anticonvulsant ( 22 ); linoleamide regulates Ca(II) fl ux ( 23 ) and inhibits the erg current ( 24 ); and erucamide stimulates Abstract Primary fatty acid amides (PFAM) are important signaling molecules in the mammalian nervous system, binding to many drug receptors and demonstrating control over sleep, locomotion, angiogenesis, and many other processes. Oleamide is the best-studied of the primary fatty acid amides, whereas the other known PFAMs are signifi cantly less studied. Herein, quantitative assays were used to examine the endogenous amounts of a panel of PFAMs, as well as the amounts produced after incubation of mouse neuroblastoma N 18 TG 2 and sheep choroid plexus (SCP) cells with the corresponding fatty acids or N -tridecanoylethanolamine. Although fi ve endogenous primary amides were discovered in the N 18 TG 2 and SCP cells, a different pattern of relative amounts were found between the two cell lines. Higher amounts of primary amides were found in SCP cells, and the conversion of N -tridecanoylethanolamine to tridecanamide was observed in the two cell lines. The data reported here show that the N 18 TG 2 and SCP cells are excellent model systems for the study of PFAM metabolism. Furthermore, the data support a role for the N -acylethanolamines as precursors for the PFAMs and provide valuable new kinetic results useful in modeling the metabolic fl ux throug...