In order to investigate the source of prolactin in amniotic fluid, immunoreactive-prolactin (IR-Prl) of decidua and villi was measured in 40 cases of early pregnancy, and analyzed by gel filtration. The IR-Prl release from these tissues was also observed in an incubation experiment. The decidua of 8 weeks of gestation contained the highest amount of IR-Prl, 81.06 +/- 2.13 ng/0.1 g dry weight. In the incubation experiment, the decidua of 12 weeks of gestation released 360.35 +/- 10.54 ng/0.1 g wet weight in 120 min. In regard to the villi, the IR-Prl concentration was low. In the profile of column chromatogram of decidua, a peak of small-Prl was observed, and the pattern of IR-Prl was similar to that of pituitary Prl. Furthermore, the IR-Prl profile was not distinguishable from the hPL fraction, but the IR-Prl and hCG profile were different from each other in the chromatogram of the extracted samples of decidua. So we concluded that the source of the Prl was decidua at least in early gestational stage.