Blood samples were collected at 0700, 1300, 1900, and 0100 hr on days 1, 7, and 20 of the menstrual cycle from women having a history of normal menstrual cycles. Plasma or serum were assayed for PGF2a; neither the stage of the menstrual cycle or the time of sample collection had any effect on the PGF2a concentration. The mean concentration of PGF2a was 0.71 ng/ml (99% confidence interval: 0.61–0.81 ng/ml, n ; 240). Collection of daily blood samples beginning 9–12 days prior to expected menstruation, and continued until 2–4 days after the onset of menses, in women with a history of dysmenorrhea showed that concentrations of PGF2a were not elevated above normal values.