We demonstrated previously that activin A released the two-cell block of mouse embryos cultured in vitro and stimulated early embryonic development. We then confirmed immunohistochemically the presence of the beta A, beta B, and alpha subunits in early embryos together with the oviductal epithelium facing those embryos. The results of in situ hybridization and reverse-transcription polymerase chain reaction here show the presence of mRNA transcripts for activin/inhibin beta A, beta B subunits in the ovary, oviduct, unfertilized egg, and embryo at the early preimplantation stage. However, the mRNA of the inhibin alpha subunit was not expressed in any of these. Considering our previous demonstration of the immunoreactive beta A, and beta B subunits of activin/inhibin polypeptides in the cytoplasm of 1- and 2-cell embryos, we suggest that activins appearing in the oviduct and in embryos are not only transferred from the follicular fluids, but produced by the oviduct, oocytes, and embryos themselves. Since the mRNA of the inhibin alpha subunit was absent at those stages, the beta A and beta B subunits may not exist as the inhibin molecule. The mRNA of activin receptor-IIB was detected in the ovary, in embryos at the 1-cell and the 8-cell/morula stages, and also in the unfertilized egg, although to a lesser extent, but not in the oviduct or in the 2-cell-stage embryo either in vivo or cultured in vitro. These results suggest that activin is physiologically involved in the process of early development of mouse embryos.