Highly sensitive and specific RIA procedures have been used to measure prostaglandin concentrations in the peripheral circulation of late pregnant and parturient women. The concentrations of prostaglandin E (PGE) and prostaglandin F (PGF) in plasma samples assayed within 4 weeks of collection were not significantly different among the groups studied, the levels (mean +/- SEM, picograms per ml) were: late pregnancy (n = 13): PGE, 4.8 +/- 1.0; PGF, 6.2 +/- 0.5; early term labor (n = 5): PGE, 6.8 +/- 1.5; PGF, 7.9 +/- 0.7; late term labor (n = 5): PGE, 5.4 +/- 2.2; PGF, 12.4 +/- 3.5; and preterm labor (n = 7): PGE, 4.4 +/- 0.4; PGF, 6.9 +/- 1.4. The concentration of 13,14-dihydro-15-keto-prostaglandin F (PGFM) in late pregnancy was 59.0 +/- 7.8 pg/ml. During spontaneous term labor, the concentration of PGFM was significantly elevated (P less than 0.01) to 142.8 +/- 32.3 pg/ml in early labor and 282.7 +/- 55.3 pg/ml in late labor. The concentration of PGFM in plasma from patients in preterm labor (62.7 +/- 17.4 pg/ml) was not significantly different from that found during late pregnancy, but was significantly lower than levels found at term during early labor (P less than 0.05). The concentration of PGE increased significantly in frozen plasma samples stored for more than 4 weeks in all groups studied; the concentration of PGF was significantly elevated after storage only in the late pregnancy group (P less than 0.01). The plasma concentration of PGFM in all groups studied was unaffected by storage. It is concluded that measurement of PGFM concentrations is the most reliable method available of monitoring prostaglandins in the peripheral circulation and that great care must be exercised in the assay and interpretation of prostaglandin levels in human plasma.