Prolactin (PRL) produced by the human uterine decidua is identical to pituitary PRL according to its chemical, biological, and immunological properties. Although much is known about the factors that modulate the pituitary production of PRL, very little is known about the control of decidual PRL synthesis and release. The studies in this thesis were undertaken to investigate the control of decidual PRL production during human pregnancy. When this study was carried out, the exact cellular origin of decidual PRL remained unclear. Using the in situ hybridization with a human pituitary PRL cDNA probe, validated by Northern blot analysis, together with immunocytochemistry to co-localize PRL mMRA and its protein product, decidual cells in the utero-placental unit were identified conclusively, for the first time, as the only source of PRL synthesis. As the decidual cells are one of the main targets of progesterone and oestrogen within the uterus, studies were carried out to examine the effect of these steroids on decidual PRL production. Firstly, the relationship between decidualization and PRL production at different stages of human pregnancy was examined using in situ hybridization and Northern blot to assess PRL mRNA level, immmunocytochemistry to examine the PRL content inside the decidual cells and radioimmunoassay to measure PRL output in amniotic fluid. The results clearly showed that PRL gene expression in human decidua paralleled the degree of decidualization. Further studies were performed to reveal how important the effect of progesterone on decidual PRL production was during early pregnancy by blocking the action of progesterone with the antiprogesterone RU486 in vivo.