Compact or cellular elastomers containing urethane and urea groups are prepared by reactinga) at least one organic and/or modified organic polyisocyanateb) at least one N-propoxylated polyoxyalkylene-polyamine containing at least 50% of secondary amino groups or a mixture of said N-propoxylated polyoxyalkylene-polyamines and polyoxyalkylene-polyamines containing 2 to 4 primary amino groups and having a molecular weight of from 1000 to 8000,c) at least one alkyl-substituted aromatic polyamine having a molecular weight up to 500, in the presence or absence ofd) catalysts and, if desired,e) blowing agents,f) auxiliaries and/org) additives.