The secretory products of the human placenta are released in the intervillous space (IS), which also represents the main site of maternal-fetal exchange. This compartment, however, has not been studied in terms of prolactin (PRL) metabolism during gestation. In the present study we determined PRL levels in maternal peripheral (MP) blood, in blood from the IS and in blood from the umbilical artery (UA) and vein (UV) of 24 clinically normal parturients and their concepti. PRL levels in MP plasma (means = 111.7 +/- 5.5 ng/ml) did not differ significantly (p greater than 0.05) from those in IS plasma (means = 131.6 +/- 64.3 ng/ml). PRL levels in UA plasma (means = 206.2 +/- 67.5 ng/ml) also showed no significant difference (p greater than 0.05) from those in UV plasma (means = 243.8 +/- 86.1 ng/ml) but both were significantly higher than MP and IS plasma levels (p less than 0.01). These data permit us to conclude that the placenta may not be the source of PRL circulating in maternal peripheral blood.