The concentrations of prostaglandin F2alpha 15-keto-13, 14-dihydro-PGF2alpha, and 13,14-dihydro-PGF2alpha were determined in plasma and in endometrial specimens taken from both dysmenorrheic and normal women at the time of their menstruation. The estimations were carried out by means of the gas chromatography-mass spectrometry method. The plasma levels of 15-keto-13,14-dihydro-PGF2alpha in dysmenorrheic women ranged between 32 and 105 pg. per milliliter, which are significantly higher than the corresponding levels in eumenorrheic women, 20 to 33 pg. per milliliter. On the first day of menstrual bleeding the concentration of PGF2alpha in the endometrium was approximately 4 times higher (300 to 2,600 pg. per milligram) in cases of dysmenorrhea than in normal subjects. Treatment with either oral contraceptives or prostaglandin synthetase inhibitors lowered the concentrations significantly and the majority of patients became symptom free.