Uteroplacental arteries of 4 rhesus monkeys were cannulated at their myometrical portions and perfused with dextran solutions and blood. Pressure-flow curves were obtained for the spiral artery and the related intervillous space, i.e. for the functional cotyledonary unit of the haemochorial villous placenta. For a flow of 3 ml.min-1, which is probably the normal rate of flow through the spiral artery, the pressure at the entry of the spiral artery was found to be 13--18 mm Hg (1.7--2.4kPa) during uterine relaxation (uterine diastole). There was a direct proportional relationship between the spiral artery flow and the pressure difference between the spiral artery and the amniotic fluid; the pressure difference per flow (the resistance of the cotyledonary unit) was found to be 1--3 mm Hg . ml-1 . min (8--24kPa . ml-1 . s). It is concluded that the spiral artery and the related intervillous space do not offer a limiting resistance to maternal blood flow in the absence of uterine contractions.

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