The racemates and the enantiomers of the imidazolidin-2-ones 2a and 2b, which can be considered as cyclic ureas, are obtained from the racemates and the enantiomers of the hydantoins la and lb by reduction with LiAlH4/AlCl3. The enantiomers that are dextrorotating in ethanol possess S-configuration. - In a study with Wistar-rats, 2a and 2b show sedative-hypnotic activity. The enantiomers exhibit marked enantioselective differences in their potency.