Cervical mucus concentrations of prostaglandins E (PGE) and F (PGF) were measured by radioimmunoassay in samples collected from 30 pregnant women. The amount of mucus that could be aspirated did not vary significantly at various stages of gestation and was 67 mg on the average. Concentrations of both prostaglandins were lowest in the first trimester and increased significantly during the second trimester. No significant further increase was observed during the third trimester. Serial samples taken from five women at approximately weekly intervals from the 35th to the 36th week onward did not show any significant increase toward term, although several individual samples had very high concentrations, suggesting that the prostaglandins were secreted into the cervical lumen in pulses. During the first and second trimester, the concentrations of PGE and PGF were similar, but during the third trimester PGF levels were significantly higher. The mean concentrations of PGE and PGF in cervical mucus during the second half of gestation were 10 to 20 times higher than their reported levels in amniotic fluid at the same stage, and in the range reported by others to be effective in modulating cervical tissue responses in vitro. Two to 4 hours after intercourse, prostaglandin concentrations in the cervical mucus were found to be about 10- to 50-fold higher than normal. The prostaglandins in cervical mucus may well participate in the gradual changes in the extracellular matrix of the cervix that result in the extensive remodeling of the cervix during pregnancy.