Transfer of fatty acids across the placenta was studied in anaesthetized rabbits at 28-days gestation by measuring umbilical venous-arterial differences, by injection of labelled palmitate into the mother and observing its appearance in the foetus, by injection of labelled palmitate into the foetus and measuring its appearance in the mother and the foetal clearance rate. 2. The release of fatty acids and glycerol by foetal adipose tissues was investigated in vitro by measuring the effect of addition of noradrenaline to the incubation medium and in vivo by measuring the effect of noradrenaline infusion into the foetus on circulating glycerol and free fatty acid concentrations. 3. In anaesthetized rabbits at 28-days gestation the maternal circulating free fatty acid concentrations were high and there was a positive umbilical venous-arterial difference. High maternal free fatty acid concentrations were associated with high umbilical venous-arterial differences. 4. Label was present in the foetus in 2 min and reached a peak in 3 min after injection of labelled palmitate into the mother. Label appeared in the maternal circulation in 1 min after injection into the foetus. The half-life of labelled palmitate was of the order of 30-60 sec in both mother in foetus. 5. Foetal white adipose tissue released both free fatty acids and glycerol into the medium and the rate of release increased four to five fold after addition of noradrenaline. Infusion of noradrenaline in the foetus led to a rise in glucose and glycerol concentrations, but the change in free fatty acid concentrations was not significant. 6. It was concluded that (i) free fatty acids can cross the rabbit placenta in amounts sufficient to provide the fatty acid components of stored triglyceride and structural lipids; (ii) placental transport of free fatty acids depends in part on maternal blood concentration and on foetal uptake; (iii) foetal circulating free fatty acids are continually exchanging with fatty acid pools in the placenta and with the maternal circulating free fatty acids.
