The influence of antioxidants on the conversion of arachidonic acid and prostaglandin endoperoxide H 2 by ram seminal vesicle microsomes was investigated. The antioxidants ascorbic acid and propylgallate enhanced significantly the formation of 6-oxo-PGF 1α. They shifted the pathway of the endoperoxide H 2 from the non-enzymic decomposition to the enzymic formation of prostacyclin. It is suggested that those antioxidants partially act through neutralisation of an oxidative species which is formed during the conversion of arachidonic acid. The other mechanisms through which they act remains to be elucidated.