Concentrations of prostaglandin E1, E2, and F2 alpha (PGE1, PGE2, and PGF2 alpha) were determined in milk and plasma from mothers of 9 preterm and 11 term infants. The concentration of PGE1 in milk was similar to that in plasma, and the concentrations of PGE2 and PGF2 alpha were approximately 1.2-2 times higher than those in plasma. However, no significant differences in the levels of PGE1, PGE2, and PGF2 alpha were observed between the foremilk and hindmilk, among the colostrum, transitional milk, and mature milk, and between preterm and term milk. Further studies have to be performed to confirm that the stable levels of PGE1, PGE2, and PGF2 alpha in human milk found in this study play an important role in the gastrointestinal function of infants.