1. The degradation of ADP by the placenta and umbilical artery was investigated. 2. Supernatants from incubations of finely chopped placental and umbilical arterial tissue were incubated with [ 14 C]ADP for various durations from 0 to 30 min. 3. Products of ADP degradation were separated by thin-layer chromatography and radioactivity incorporated into each product was measured. 4. Placental supernatants induced a more rapid degradation of ADP than the umbilical artery supernatants. The main product of ADP degradation by placental supernatants at 30 min was adenosine, whereas that of umbilical artery was AMP. 5. This conversion by placenta of ADP, a potent platelet aggregator and vasoconstrictor, into adenosine, a potent platelet anti-aggregator and vasodilator,-5-be important in the maintenance of perfusion of the foetoplacental unit.

1. The rate of decay of the platelet anti-aggregatory activity of prostacyclin in plasma and buffer solutions was investigated. 2. Platelet anti-aggregatory activity of prostacyclin was more stable in plasma than in buffer solutions at physiological pH. 3. The rate of decay of this activity increased with an increase in incubation temperature. 4. Preheating of plasma at 65 °C led to a marked but not total loss of its ability to ‘protect’ prostacyclin. 5. Incubation of 6-keto-prostaglandin F 1α in plasma did not result in the appearance of platelet anti-aggregatory activity; nor did the decay curves of prostacyclin activity indicate the emergence of any new platelet anti-aggregatory activity. 6. Incubation of 6-keto-prostaglandin E 1 in plasma did not lead to any loss of its platelet anti-aggregatory activity for prolonged periods. 7. We conclude that the marked prolongation of platelet anti-aggregatory activity of prostacyclin in plasma is not due to its enzymatic conversion into 6-keto-prostaglandin E 1 but due to ‘protection’ of prostacyclin itself. This ‘protection’ is probably due to an interaction with a plasma protein.