The invention relates to a process for the solventless production of elastic filaments based on segmented poly(urethane)s or poly(urethane urea)s having improved thermal properties and, hence, improved processing behavior, for example during the heat setting of fabrics containing the elastic fibers, by the use of a polyisocyanate containing isocyanurate groups in the reaction of long-chain, relatively high molecular weight polyhydroxyl compounds with organic diisocyanates and low molecular weight diols and, optionally, other auxiliaries and additives in the melt, direct spinning of the reaction mixture thus obtained and thermal aftertreatment of the elastic filaments obtained and also to the filaments obtainable by this process.