Abstract. The cellular origin of placental hCG is not yet completely established. Depending on the method used, the syncytium, cytotrophoblast or both types of tissue have been claimed to synthesize hCG. In the present study in situ hybridization was used on sections of chorionic villi in order to detect expression of the gene for the β-subunit of hCG (β-hCG). Placental tissue was obtained from the 8th to the 11th weeks of pregnancy, when the concentration of hCG is high. A cDNA clone, encoding the entire amino acid sequence of β-hCG, was used as a probe. Hybrids of β-hCG cDNA/mRNA were found only over the syncytiotrophoblast. Background noise was extremely low and no signals above that were detected in the cytotrophoblast cells. It is concluded that at this stage of pregnancy the gene for β-hCG is activated in the syncytial parts of chorionic villi and not in the cytotrophoblast.