The serum level of unconjugated 17 beta-oestradiol (E2) and oestriol (E3) in the maternal vein (MV), the umbilical vein (UV) and artery (UA) immediately after term (n = 34) and preterm (n = 74) labour was measured to clarify the hormonal changes that occur between the maternal and fetal compartments. The following results were found. (1) The level of E2 and E3 increased equally in the MV, UV and UA serum from the 28th-32nd week to the 33rd-36th week of pregnancy. From the 33rd-36th week to the 40th week there was no change in the MV, but the value of E2 and E3 decreased significantly in the UV and UA serum. (2) The serum level of E2 in the MV was significantly higher than that in the UV and UA during every gestational period. In contrast, the serum concentration of E3 in the MV was significantly lower than that in the UV and UA. (3) The value of 'UA/UV X 100' of E2 and E3 was about 30% during the 28th-40th week. (4) A weak correlation was found between the MV serum level of E2 and E3 and UA serum concentrations. A strong correlation was found between the UV and UA serum levels of E2 and E3. The authors suggest that though there is a close connection between the fetal and the maternal organism, the fetus is still capable of maintaining its hormonal environment independently.