Apelin was thought to be an adipocyte-specific hormone, but recent studies have indicated a link between apelin and placenta function e.g. cell proliferation. The aim of the study was investigating dose- and time-dependent effect of apelin on hormone secretion including steroids: progesterone (P4) and estradiol (E2) and proteins: chorionic gonadotropin (hCG), human placental lactogen (hPL), placental growth factor (PLGF), as well as protein expression of steroid enzymes (3βHSD, CYP19) and protein hormones (hCG, hPL and PLGF) in placental cells. Syncytiotrophoblast BeWo cells, as human trophoblast models, were treated for 24, 48, and 72 hours with the human recombinant apelin at doses 0.02, 0.2, 2.0, 20 and 200 ng/ml followed by culture medium. Concentrations of the above hormones were studied by ELISA kits. Furthermore, protein expression of steroid enzymes and protein hormones were measured using Western blot. Our results showed that apelin significantly decreased both steroid and protein hormones by inhibiting steroid enzymes or protein hormone expression. Moreover, we demonstrated that apelin at dose 2.0 ng/ml increased phosphorylation of protein kinase A (PKA) from 1 to 60 min of BeWo cell incubation. Inhibitory effect of apelin on P4, E2 and PLGF secretion were abolished when BeWo cells were cultured in the presence of ML221, an apelin receptor antagonist, PD98059, an extracellular signal-regulated kinases (ERK1/2) antagonist and KT5720, a PKA antagonist. In turn, secretion of hCG and hPL occurs only in the presence of ML221 and PD98059. In conclusion, our results indicate that apelin can be considered as a gestational hormone implied in the endocrine function of the human placenta, with an important role in controlling the production of steroid and protein hormones in placental BeWo cells.