The influence of antioxidants on the conversion of arachidonic acid and prostaglandin endoperoxide H2 by ram seminal vesicle microsomes was investigated. The antioxidants ascorbic acid and propylgallate enhanced significantly the formation of 6-oxo-PGF1.alpha.. They shifted the pathway of the endoperoxide H2 from the non-enzymic decomposition to the enzymic formation of prostacyclin. Those antioxidants may partially act through neutralization of an oxidative species which is formed during the conversion of arachidonic acid. The other mechanisms through which they act remain to be elucidated.