1. The effect of free radical (superoxide and hydroxyl) and hydrogen peroxide scavengers on the formation of prostaglandin E2 by ovine placental microsomes was investigated. 2. Of the free radical scavengers tested only Mn Desferal, a low molecular weight (712 Da) superoxide scavenger, at concentrations of 0.5-5 mmol/1 significantly (P < 0.001) stimulated PGE2 formation by ovine placental microsomes. 3. This observation is consistent with the involvement of superoxide radicals in the inactivation of prostaglandin G/H synthase (PGHS). 4. The results obtained further suggest that if superoxide radicals are important regulators of PGHS activity in the ovine placenta, they are not accessible to large molecular weight free radical scavengers.