The study estimated the mean volume of the clefts between adjacent villi in the normal term human placenta, and the effect on this of increasing the fetal perfusion pressure, using a new stereological tool called the 'star volume estimator'. This enables the measurement of irregular and complex structures, including both particles and cavities, in a mathematically defined and unbiased manner. To achieve this, a total of ten term placentae delivered by caesarean section were obtained. Four fetal arteries supplying opposite quadrants of the placental disc were perfusion-fixed under standard pressures of 40, 60, 80 and 100 mmHg respectively. Stereological estimates relating to the star volume of the clefts between the villi, and to the volume density of the intervillous space were obtained. There was a significant rise in the star volume of the intervillous clefts from 26.8 x 10(4) m3 at 40 mm Hg to 75.1 x 10(4) m3 at 100 mmHg (F = 75, df = 1.38, P < 0.05). The volume density of the intervillous space remained unchanged, thus obviating the possibility of fluid leakage into the intervillous space accounting for this change. It is concluded that the fetal vasculature provides hydraulic support to the villous tree, and that changes in the umbilical perfusion pressure can therefore alter the disposition of the villi within the intervillous space. As fetal blood pressure rises, for example during acute hypoxic episodes, the villi will move apart. The enlargement of the clefts between adjacent villi will have a secondary effect upon the maternal circulation, promoting more even perfusion of the intervillous space at higher overall flow rates.(ABSTRACT TRUNCATED AT 250 WORDS)