Arachidonic acid metabolism and prostanoid synthesis were investigated using [14C]-arachidonic acid (AA) in human endometrium obtained at different phases of the ovarian cycle and decidua in early pregnancy. Prostaglandins (PG) identified in endometrium and decidua were PGE2 and PGF2 alpha. The capacity for the production of PGE2 and PGF2 alpha was higher in the secretory phase of endometrium than in the proliferative phase, and the maximum formation of PGE2 and PGF2 alpha was found in the mid secretory phase and in the late secretory phase, respectively. However, the production of PGs was markedly suppressed in decidua. On the other hand, more than 70% of [14C]AA was incorporated into total neutral lipids, whereas less than 22% was incorporated into glycerophospholipids in both endometrium and decidua. Secretory endometrium showed a higher uptake of [14C]AA into glycerophospholipids than did proliferative endometrium, whereas the incorporation into neutral lipids was lower in secretory endometrium than in the proliferative phase. Moreover, decidua showed a similar pattern of incorporation of [14C]AA to the proliferative phase. The changes of arachidonic acid metabolism and PG formation in both endometrium and decidua are discussed in terms of implantation and maintenance of early pregnancy.