The production of protein hormones by human placenta, amnion, chorion and decidua capsularis was studied in in vitro experiments to establish whether the decidua could be considered a specialized structure for the release of PRL [prolactin]. The tissues were incubated in vitro and the release of PRL. GH [growth hormone] chorionic somatomammotropin [CS] and CG [chorionic gonadotropin] was investigated. In the incubation medium, a highly significant rise of PRL during culture of the decidua was found, while no increase was noticed during culture of the placenta or amnion. CS and CG increased greatly and quickly during culture of placenta but not during culture of other tissues. No significant change was found in GH. The total PRL released into the medium from decidua was 3 .times. higher than the initial PRL content of this tissue; addition of puromycin to the incubation medium reduced both the tissue content and the release of PRL to almost 50% of the control values. This result raises the possibility of a specific endocrine activity of decidua capsularis. The PRL-secreting cells of the decidua probably do not possess dopamine receptors, since bromocriptine, when added to the medium, did not influence the release of PRL, confirming previous in vivo observations.