1. Permeability of the term human placenta to inulin and mannitol was measured in six normal pregnancies. 2. Bolus and continuous infusion of inulin and mannitol to the mother for 1-2 h before delivery by Caesarean section achieved steady-state maternal serum concentrations. Net accumulation of inulin and mannitol in the fetus was measured by the subsequent neonatal urinary excretion. 3. Transplacental clearance was calculated by dividing the net flux by the time-averaged transplacental concentration gradient. Clearance for inulin was 0.86 +/- 0.1 ml min-1 (mean +/- S.E.M.) and for mannitol 7.3 +/- 0.8 ml min-1 (n = 6 in both cases).