N-Sulphamoylphenyl-substituted imidazolidin-4-ones having useful anti-epileptic activity are prepared by cyclising N-sulphamoylphenyl-substituted .alpha.-aminoacetamides with formaldehyde or a reactive derivative of carbonic acid, or by reacting N-halosulphonylphenyl-substituted imidazolidin-4-ones with ammonia.