A relationship between the menstrual cycle, changes of bowel habits and concentrations of plasma prostaglandin (PG)E2, PGF2 alpha, 6-keto-PGF1 alpha and thromboxane (TX)B2 in 3 groups of 8 women with different bowel habit were determined. The concentrations of PGE2, PGF2 alpha and TXB2 were significantly higher in the group who had bowel habits smoother than usual at menses compared to those who had experienced constipation throughout cycle or at menses. However, no differences between at mid-cycle and at menses were observed in the groups who had experienced constipation. These results suggest that constipation of young women is related to the inherited inability of patients to synthesize and secrete PGs in plasma and possibly in small intestine.