Low-shrinkage nylon moldings are produced by activated alkaline lactam polymerization, preferably using the reactive injection molding technique, by a process wherein the polymerization is carried out in the presence of from 5 to 80% by weight of a polymer P which1. is completely soluble in the lactam,2. separates out in the course of polymerization at a conversion of from 1 to 50%,3. is composed of one or more blocks which are compatible with polylactam and one or more blocks which are incompatible therewith, and preferably4. possesses functional groups which are incorporated into the growing polylactam chain.