Prostaglandins have been detected in high levels in the fetal circulation. These substances are known to cause marked changes in the fetal and uterine circulations. The present study demonstrated an increase in fetal and maternal circulating levels of i prostaglandin F2 alpha in fetal lambs with acute acidosis. Both levels correlated with the degree of fetal acidosis. No consistent change in levels of i PGE were noted. Prostaglandins may play a role in the pathogenesis of response to fetal stress.