In preimplantation stages of normal and spontaneously activated parthenogenetic embryos of the LT/Sv mouse strain, protein synthesis was analyzed by using two-dimensional polyacrylamide gel electrophoresis. Fertilization and parthenogenetic activation cause similar changes polypeptide synthesis when compared with those of unfertilized eggs. The overt developmental delay of early parthenotes, which is probably due to an initial retarded activation in comparison with normal fertilization, is documented molecularly by a similar delay in their protein synthesis pattern. These differences are clearly visible at the two-cell stage but gradually disappear during further cleavage. The basic protein patterns of normal and parthenogenetic embryos are remarkably similar up to the blastocyst stage. However, quantitative differences occur in all preimplantation embryos analyzed and become more distinct at the blastocyst stage. In addition, only minor qualitative changes appear during late preimplantation. These alterations in protein synthesis may reflect at the molecular level early events in abnormal development of parthenotes. Our biochemical results are discussed in context with biological experiments rescuing parthenogenetic LT/Sv embryos by chimera formation.