The capacity of the freshly delivered human term placenta to produce and release placental proteins during in vitro dual perfusion was investigated. The organ was perfused in separate closed circulations and aliquots of medium were taken at regular intervals from both maternal and fetal circuits. The placental proteins human chorionic gonadotrophin (HCG), human placental lactogen (HPL), pregnancy-specific β1-glycoprotein (SP1), and pregnancy-associated plasma protein A (PAPP-A) were quantified in these media as well as in the placental tissue before and after the perfusion. It was found that the four above-mentioned proteins were synthesised during the perfusion interval (90 min to 3 h) while pregnancy-associated α2-glycoprotein and prolactin were only washed out. The mean production of HCG, HPL, SP1, and PAPP-A was decreased when either cycloheximide, puromycin, iodoacetic acid, or 2,4-dinitrophenol had been added to the perfusing medium. Amongst these four antimetabolites iodoacetic acid most severely affected both the total release and net synthesis. It is concluded that the above four proteins are synthesised de novo by the perfused placenta in the absence of maternal tissue and that this synthesis is energy-dependen