We studied the expression of mRNA encoding the a- and β-subunts of marmoset chrrionic gonadotrophin (mCG) in implantation stage blastocysts and in a tropho-blastic cell line derived from such blastocysts. In this investigation in situ hybridization was carried out using digoxygenin-labelled riboprobes to localize the subunit transcripts. The trophoblastic cell line, known to secrete bioactive mCG, was used as a positive control. Marmoset uterine embryos were cultured to hatched blastocysts and following growth on Matrigel or plastic were processed for in situ hybridization at developmental stages ranging from 13-15 days post-conception. In serial sections mCG-β mRNA was detected mainly in polar, trophoblast. The mRNA for mCG-a was expressed more uniformly in polar and mural trophoblast. Transcripts for the β-subunit were not expressed, or present as weak signals, in the inner cell mass (ICM) and endoderm. However, low levels of mRNA for mCG-a were detected in the ICM and visceral endoderm. We have concluded that mRNA for mCG-β was primarily localized to patches of syncytiotrophoblast at the embryonic pole and sparsely distributed in mural trophoblast, while the transcripts for mCG-a were distributed more uniformly in differentiating cytotrophoblast and syncytium, and at much lower levels in ICM and early endoderm.