Levels of prostaglandin E2, prostaglandins F2 alpha and prostacyclin (measured as 6-keto PGF1 alpha) were measured by radioimmunoassay in aliquots of foremilk and hindmilk obtained at different stages of lactation, (colostrum, transitional and mature milk), from ten healthy nursing mothers who delivered at term. Immunoreactive prostaglandins E2, F2 alpha and the stable metabolite of prostacyclin 6-keto PGF1 alpha were detected in all fresh human milk samples but not in cow's milk-based formulas. The source of prostaglandins and prostacyclin in breast milk is related to local synthesis of the mammary gland and to the synthesis by the cellular elements of breast milk but not to their level in the maternal circulation.