Prostaglandins Leukotrienes and Medicine 14: 199-204, 1984 PROSTAGLANDIN SYNTHASE INHIBITORY ACTIVITY IN THE PLASMA OF RHESUS MONKEYS DURING LATE PREGNANCY: EFFECT OF DEXA- METHASONE Murray D. Mitchell’, Shaun P. Brennecke’ and Miles J. Novy3 ‘Cecil H. and Ida Green Center for Reproductive Biology Sciences and the Departments of Biochemistry and Obstetrics and Gynecology; The University of Texas Southwestern Medical School, 5323 Harry Hines Boulevard, Dallas, Texas 75235 [Reprint requests to MDM] 2 Nuffield Department of Obstetrics and Gynaecology, University of Oxford, Oxford, England. Present Address: Department of Obstetrics and Gynaecology, Queen Elizabeth Hospital, Woodville, South Australia 50011 AUSTRALIA 3 Division of Reproductive Biology and Behavior, Oregon Regional Primate Research Center, 505 N. W. 185th Avenue, Beaverton, Oregon 97006 ABSTRACT Plasma from rhesus monkeys during late pregnancy contained a factor(s) that was inhibitory of prostaglandin synthase activity. There was no consistent trend in the inhibitory activity of plasma obtained between 120 days of gestation and term (approximately 167 days of gestation I. Administration of dexamethasone (0.125 to 1 .O mg twice daily) consistently increased inhibitory activity in plasma after an initial treatment period of S-10 days. Administration of dexamethasone for several weeks is associated with prolongation of gestation in rhesus monkeys. We suggest that the action of dexamethasone, to delay parturition is related, in part, to an enhancement of the activity of a circulating factor(s) that is inhibitory of prostaglandin synthase activity. INTRODUCTION Blood plasma and serum of several mammalian species can act to inhibit prostaglandin synthase (1,2). These findings are suggestive of the presence in blood of an endogenous inhibitor(s) of prostaglandin synthase (EIPS). Circulating EIPS may be a systemic arm of the body’s regulatory mechanisms which govern prostaglandin biosynthesis. 199