Differences in prostaglandin (PG) F2 alpha synthesis and degradation were sought between early luteal endometrium (histology day 15 to 22, n = 6) and late luteal endometrium (histology day 23 to 28, n = 6). In addition, alterations in PGF2 alpha synthesis and degradation in response to 17 beta-estradiol (E2) and progesterone (P) were examined to clarify the mechanism of steroid modulation of endometrial PG production (n = 12). Endometrium was maintained in tissue culture and the concentration of PGF2 alpha and 13,14 dehydro-15 keto F2 alpha (DHKF2 alpha) was determined in spent media by radioimmunoassay. Prostaglandin F2 alpha and DHKF2 alpha output from luteal endometrium exposed to P and E2 + P were both significantly reduced when compared with no addition or E2 treatments. This implies that the modulation of PGF2 alpha output by P in vitro is secondary to altered synthesis. There was an increase in PGF2 alpha output from late luteal endometrium when compared with early luteal endometrium in the P and E2 + P treatments, but DHKF2 alpha remained unchanged. These data imply that the temporal increase in PGF2 alpha output is the result of alterations in both PG synthesis and catabolism.