- Isoprostanes are members of a family of prostaglandin isomers that are produced by free radical-catalysed mechanisms. They have become well-recognized indicators of oxidant-induced cell damage in a variety of pathophysiological conditions. Several isoprostanes have been shown to possess biological activity in whole-animal, isolated tissue and cell-based systems. Their actions include vasoconstriction, platelet aggregation and cardiac hypertrophy. Current evidence suggests that these effects are mediated by prostanoid receptors through a complex set of interactions that involve agonism, partial agonism, desensitization and co-operative behaviors. It is likely that other mechanisms of action are waiting to be discovered. Based on a consideration of these biological effects, we argue that isoprostanes are more than mere markers and may serve as active participants in promoting and exaggerating pathophysiological changes. To tease out their roles requires considerable more work and a willingness to suspend disbelief based on limited evidence.