PIP: The serum prostaglandin (PG) F2alpha levels of women in the last trimester of pregnancy have been measured and compared with the levels found in samples taken during labor, during the 1st week postpartum, and from nonpregnant women. The laboratory procedures for drawing the samples and making the radioimmunoassay measurements are explained. 40 samples were taken during late pregnancy, 46 during labor, 7 during the 1st week postpartum, and 8 from nonpregnant women. The results are graphed. PGF2alpha levels during late pregnancy resemble those found in nonpregnant women. A significant rise (p.05) occurs during labor, most markedly during the late 1st stage. Levels during the 1st week postpartum were found to be very low. There was no difference between serum levels during the 33rd-36th weeks and those found during weeks 34-40. Serum levels did not differ according to whether labor was spontaneous, induced by artificial rupture of fetal membranes and infusion of Syntocinon, or merely accelerated by syntocinon infusion. This indicates that PG release occurs as a result of labor rather than as a triggering mechanism for the onset of labor. The highest levels of serum PGF2alpha seem to be associated with the time of greatest uterine activity and the period of greatest cervical stretching.