Compact or cellular elastomers containing urethane and urea groups are prepared by reacting a) at least one organic and/or modified organic polyisocyanate b) at least one N-propoxylated polyoxyalkylene-polyamine containing at least 50% of secondary amino groups or a mixture of said N-propoxylated polyoxyalkylenepolyamines 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 of d) catalysts and, if desired, e) blowing agents, f) auxiliaries and/or g) additives.