Permeability of the human placenta in vivo to four non-metabolized hydrophilic molecules.

1. Permeability of the human placenta to four permeants of different molecular size was measured at Caesarean section in seven normal full-term pregnancies. 2. Placental clearance for mannitol was 8.7 +/- 1.1 ml min-1 (mean +/- S.E.M.), lactulose 6.3 +/- 0.8, chromium ethylenediaminetetraacetic acid (CrEDTA) 3.7 +/- 0.5, and inulin 0.98 +/- 0.1. 3. The permeability data were analysed in terms of restricted diffusion through 'notional' water-filled pores and found to be incompatible with a single population of uniform pores.