1. The placental transfer of urea was studied by perfusing the guinea-pig foetal placenta in situ with dextran solutions containing various amounts of urea, and radioactively labelled urea.

2. Transfer of urea was linearly related to the difference in concentration between the maternal and the foetal sides of the placenta, but transfer in both directions across the placenta was equal when the concentration of urea in the perfusing fluid was 2.5–3.5 mmol/l less than the maternal arterial value. This suggested that urea may be transferred against a concentration gradient.

3. Foetal plasma urea concentrations were found to be 0.5 mmol/l less than the maternal, suggesting that active transfer from the foetal circulation to the maternal can occur. However, because of the close relationship between foetal and maternal plasma urea (r = 0.96), it is concluded that the major control of foetal urea concentrations is by diffusion of urea between maternal and foetal extracellular fluids.

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