The demonstration of the essential role of fatty acid amide hydrolase (FAAH) in hydrolyzing endogenous bioactive fatty acid derivatives has launched the quest for the discovery of inhibitors for this enzyme. Along this line, a set of 58 imidazolidine-2, 4-dione and 2- thioxoimidazolidin-4-one derivatives was evaluated as FAAH inhibitors. Among these compounds, 3-substituted 5, 5'-diphenylimidazolidine-2, 4-dione and 3-substituted 5, 5'- ...