The concentrations of prostaglandin E (PGE), prostaglandin F (PGF) and 13,14-dihydro-15-oxo-PGF (PGFM) have been measured by sensitive and specific radioimmunoassays in neonatal plasma after term and pre-term delivery. Blood samples were taken in the term delivery group from the umbilical artery at birth and on the sixth post-natal day and after pre-term delivery at 2-4 days, on the sixth day, at 2-4 weeks and at 5-8 weeks after birth. The levels of prostaglandins circulating during the first month of life were far greater than those found in normal adults. In neonates delivered at term the plasma concentration of PGE was significantly lower six days after delivery compared with the concentration at delivery whereas the concentrations of PGF and PGFM were essentially unchanged. Following pre-term delivery prostaglandin concentrations declined with increasing neonatal age although only levels of PGE at 5-8 weeks of age were within the normal range of adult values. Comparison of prostaglandin levels six days after delivery between neonates born at term and pre-term showed no significant differences. These results suggest that prematurity per se is not associated with marked abnormalities in the ability of the neonate to synthesize or metabolize prostaglandins.