JAR choriocarcinoma cells have retained several characteristics of normal trophoblasts and have been used as an in vitro trophoblast model. The superfusion system is suitable for the study of hormone synthesis and/or secretion. JAR cells were cultured and transferred to the superfusion system in order to evaluate the spontaneous hCG secretion and the effect of GnRH. The spontaneous hCG release showed a periodic pattern with a 48 min phase interval. In our system single cells and cell-clusters were superfused and there is a possibility that cell to cell connections might have an influence on the regulation of hormone synthesis and/or secretion. GnRH in 4 x 10(-7) M and 4 x 10(-6) M concentrations or 100 mM KCl caused an immediate hCG release from the JAR cells Repeated administration of GnRH resulted in a delayed hCG release which is probably related to the relatively small amount of hCG available from the storage granules or to the phase of spontaneous secretion.