1. Portal hypertension was produced experimentally in rats by partial ligation of the portal vein.
2. One week later portal veins of these animals were shown to release significantly greater amounts of prostacyclin than control animals.
3. It is postulated that if this response occurs in man, local vasodilatation and inhibition of platelet aggregation may be exacerbatory factors in the severity of haemorrhage from oesophageal varices that complicate portal hypertension.