This study was conducted to examine the release of immunoreactive prolactin (IR-PRL) in vitro by 14 cases of decidual obtained from women with 6 to 13 weeks of gestation. 1) The dilution curve of IR-PRL released from the decidua was parallel to the pituitary standard PRL. 2) A highly significant rise of IR-PRL during the incubation of the decidua was found. In decidua of 11-13 weeks of gestation, the summarised IR-PRL level during 90 minutes was the highest, viz. 359.4 +/- 87.4 ng/0.1 g wet weight. 3) The villi showed scarcely any IR-PRL release, and the accumulated IR-PRL level was 2.4% of the decidual incubation value. Little IR-PRL release was found in the incubation of endometrium of luteal phase. 4) The temperature and pH of the medium influenced the IR-PRL release, but dopamine, bromocriptine and somatostatin had no effects on the PRL releasing activity of the decidua. 5) According to the gel filtration, the elution pattern of IR-PRL released from the decidua was similar to that of the amniotic fluid rather than that of the maternal plasma. From the above results, we suggested that the decidua was the original source of the PRL released in the amniotic fluid.