The human placenta and its associated membranes are vital to the maintenance, nutrition, and protection of the developing fetus. During placental development some cytotrophoblasts give rise to the chorionic membrane whereas others fuse to form a differentiated syncytium of cells that are responsible for placental protein and steroid hormone production. The mechanisms involved in the differentiation of the trophoblasts are unknown; however, an intermediate stage with a characteristic phenotype has been documented in vivo. We have observed that chemically dissimilar xenobiotic agents induced BeWo choriocarcinoma cells to change their usual cytotrophoblastic phenotype and acquire morphologic and functional characteristics typical of intermediate trophoblast. Incubation of BeWo cell cultures in the presence of 1 mumol/L methotrexate for 48 hours stimulated human chorionic gonadotropin secretion (2.3-fold) and aromatase activity (4.9-fold). Morphologic findings associated with these hormonal changes, including increased nuclear size and cytoplasmic expansion, were also observed. With the use of a computer-interfaced image analyzer, planimetric morphometry of the nuclear area of the cells revealed a 1.8-fold increase after incubation with methotrexate. The effect of forskolin, a direct activator of adenylyl cyclase, was also evaluated by means of this model of cytotrophoblast differentiation. The addition of 10 mumol/L forskolin to BeWo cultures also resulted in dramatic changes in trophoblast cell phenotype. Increases in human chorionic gonadotropin synthesis (7.3-fold), aromatase activity (13.5-fold), and nuclear area (3.0-fold) were induced over those of untreated cells. In addition, increases in [3H]thymidine incorporation (1.7-fold) were afforded by both treatments. These results suggest that biochemical and cytologic changes associated with human trophoblast differentiation can be induced in vitro via activation of the adenylyl cyclase pathway by forskolin and through unknown and apparently independent signals by methotrexate.