The capability of cytotrophoblast cells to produce hCG, progesterone, estrogen, cGnRH and beta-endorphin in vitro has been demonstrated in serum-free culture medium. Before experiment, a 24-h preculture was carried out in order to remove the endogenous hormones of the tissue. During a period of 8 days' culture, the cytotrophoblast cells could constantly produce a small amount of hCG. The production of progesterone rose rapidly and became doubled within six days. The estrogen secretion showed a similar pattern in the presence of androstenedione, a precursor of estrogen, indicating the elevation of aromatase activity in the cells. The elevation of the enzyme activity has been further demonstrated not to be induced by androstenedione. In both cytotrophoblast and syncytiotrophoblast cell cultures, cGnRH was only detected in the culture of cytotrophoblast cells, with a value up to 4 pg/10(5) cells/24 h. However, beta-endorphin was identified both in cytotrophoblast and syncytiotrophoblast cells. Its content increased significantly in the medium of cytotrophoblast cell culture from the 4th to 6th days, but declined in the medium of syncytiotrophoblast cell culture. The results demonstrate clearly that the cytotrophoblast cells are the sole origin of GnRH in human placenta and are also able to synthesize beta-endorphin and steroid hormones. The findings indicate that there is no such a sharp functional demarcation existing between these two kinds of trophoblast cells as suggested before. The data are of significance for a better understanding of the mechanism of hormonal regulation in placenta.