Imidazolidinone derivatives of the formula ##STR1## where R, R.sub.1, R.sub.2 and R.sub.3 independently are hydrogen or lower alkyl, X represents one or two substituents selected from the group lower alkyl, halo, lower alkoxy, trifluoromethyl, and their pharmaceutically-acceptable acid salts. The compounds are prepared essentially by the method described by Wright in J. Med. Chem. 9, 852 (1966). The compounds have high depressant activity on the central nervous system.