Menstrual fluid was collected in vaginal cups inserted for 2 h during the first 2 days of menstruation and menstrual serum concentrations of prostaglandins PGF2 alpha and PGE2 were measured by radioimmunoassay. In 16 women from whom menstrual fluid was collected on both days, PGF2 alpha and PGE2 concentrations were significantly higher on day 1 than on day 2. The highest concentrations of PGF2 alpha and PGE2 were found in dysmenorrhoeic women on day 1. In non-dysmenorrhoeic women, the amount of PGF2 alpha and PGE2 collected in 2 h correlated directly with total menstrual blood loss. There was no statistically significant difference in the amount of prostaglandins collected in 2 h in pain-free menorrhagic women and dysmenorrhoeic women with normal loss. There was also no significant 9-ketoreductase or 9-hydroxydehydrogenase activity present in menstrual fluid which could suggest PGE2 to PGF2 alpha interconversion.