Retinoic acid (RA), an active metabolite of vitamin A, is an important mediator of cellular differentiation and has been shown to stimulate human CG (hCG) secretion by JEG-3 choriocarcinoma cells in vitro. In order to determine whether RA stimulates the hCG secretion by other trophoblastic cell lines, we evaluated the effect of RA on hCG, hCG-alpha subunit (hCG-alpha), and progesterone secretion in three choriocarcinoma cell lines: JEG-3, JAR, and BeWo. RA stimulated hCG and hCG-alpha secretion in a dose-dependent fashion by each of the three cell lines. The time required to give a statistically significant increment of hCG and hCG-alpha over control cells was 48 h. The addition RA to cholera toxin (10 micrograms/ml) resulted in an additive or synergistic stimulation of hCG and hCG-alpha secretion by the three cell lines. Cycloheximide (1 microM) abolished the effect of RA on hCG and hCG-alpha secretion in the BeWo cell line. Progesterone secretion in response to RA was inconsistent. Progesterone secretion by both JEG-3 and BeWo cell lines were stimulated at high concentrations of RA (1 x 10(-6], whereas progesterone secretion by JAR cells was not stimulated. Intracellular levels of cAMP were not affected by RA treatment in the JEG and JAR cells. In the dosages used, RA did not significantly alter cell number in any of the cell lines. RA in physiologic concentrations stimulates hCG and hCG-alpha secretion by three choriocarcinoma cell lines in vitro. Whether RA is a physiologic mediator of placental hormone production is unknown.