This investigation of trophoblasts (Trs) in early gestation was performed by histologic and immunohistochemical evaluation of the Trs of the basal plate, decidua and myometrium, in an attempt to determine the changes in structure and function dependent on their location. The structural evaluation was performed by both light and electron microscopy and the functional evaluation by the localization of hCG, its subunits and hPL. In the syncytiotrophoblastic cell layer both hCG, its subunits and hPL are localized intracellularly. In the decidua, the so-called invasive Trs, only hPL could be detected. Histologically, the Trs in the deeper decidua tend to coalesce and become multinucleated. Thus, villous Trs and invasive Trs differ not only structurally, but also functionally, as can be seen by the differences in the synthesis of hCG and hPL, leading to the assumption that Trs have varying functions dependent upon their location.