New 1-(1,5-dimethyl-3-oxo-2-phenyl-2,3-dihydro-1H-pyrazol-4-yl)-2-thioxo-3-aryl/cyclo-hexyl-4,5-imidazolidinediones 2a-g, 1-(2-benzoyl-4-chlorophenyl)-2-thioxo-3-phenyl-4,5-imidazolidinedione 4 and 1-(substituted methylidene)amino-2-thioxo-3-cyclohexyl-4,5-imidazolidinediones 6a-k are synthesized from N-(1,5-dimethyl-3-oxo-2-phenyl-2,3-dihydro-1H-pyrazol-4-yl)-N -aryl/cyclohexylthioureas 1,. N-(2-benzoyl-4-chlorophenyl)-N-phenyl-thiourea 3 and I-substituted methy lidene-4-cyclohexylthiosernicarbazides 5, respectively. The structures of the compounds are determined by analytical and spectral (IR, H-1 NMR, EIMS) data. The synthesized compounds are evaluated for their potentiating effects on pentobarbital induced hypnosis in mice. All of the compounds increase the sleeping time of pentobarbital significantly (p<0.001) and the most potent compound is found to be 6d.