A polarographic oxygen electrode technique was used to quantify the activity of prostaglandin G/H synthase present in microsomes prepared from cotyledons of ewes at 74 to 147 days of gestation (n = 25). Microsomal oxygen consumption was stimulated by arachidonic acid and suppressed significantly (p less than 0.0001) by indomethacin, a specific inhibitor of prostaglandin G/H synthase. A residual indomethacin-incentive oxygen consumption was identified but the magnitude of this component did not change during gestation. During gestation prostaglandin G/H synthase (PGHS) activity, present in ovine cotyledons, increased 22-fold, from 0.6 +/- 0.19 at 70-80 days (n = 3) to 13.22 +/- 2.08 nmol O2/min/mg protein at greater than 145 days of gestation (n = 8; p less than 0.001). This gestational increase in the PGHS activity present in ovine cotyledons was not observed when fetal hypophysectomy was performed at 70-80 days of gestation. The data obtained in this study are consistent with the hypothesis that cotyledonary PGHS activity is increased during the second half of gestation and that this increase may be dependent upon fetal pituitary factors.