The n-acylethanolamides (NAEs) are a family of naturally Occurring monoethanolamide containing lipids that display a variety of interesting biological properties. In this Study, some physicochemical properties of a series of saturated monoethanolamide lipids with increasing hydrocarbon chain length (lauroyl, myristoyl, palmitoyl, and stearoyl) have been investigated. Temperature induced phase transitions for these NAEs indicate that both the monoethanolamide headgroups and the unsaturated hydrophobic tails play a role in the melting behavior of these lipids. All four lipids examined demonstrate the presence of at least three different polymorphic crystal forms. Transitions in crystal Structure call be induced via heating and Visualized with polarized optical microscopy. At room and physiological temperature, the four NAEs are solid lamellar crystalline materials. All four molecules form lyotropic liquid crystalline phases in water, albeit at relatively high temperatures, including the lamellar liquid crystalline phase and at least two isotropic phases. Lamellar crystalline palmitoyl monoethanolamide was dispersed as solid lipid nanoparticles (SLNs). The cytotoxicity of these SLNs toward human mammary epithelial cells (HMEpiC) and the MCF7 breast cancer cell line was assessed at physiological temperature. The palmitoyl monoethanolamide SLNs showed little to no toxicity to the HMEpiC even at a concentration of 30 mu M. At concentrations above 3 mu M, the HMEpiC population was reduced by less than 15%, while the MCF7 population was reduced by similar to 20-30%. The endogenous nature and natural medicinal properties make this series of lipids ideal candidates for further investigation as solid lipid nanoparticle drug delivery systems.