Peripheral plasma prostaglandins (PGs) were assayed in 10 cases of gynecologic malignancies. In addition, fluctuations of PG levels during chemotherapeutically-induced gastrointestinal toxicity as well as those caused by a bolus infusion of steroid hormone were investigated. As a result, the level of PGE2 in most cases of gynecologic malignancies was seen above or around the upper limit of that in healthy women. During chemotherapy, the levels of PGF2 alpha and thromboxane B2 (TxB2) increased significantly compared to baseline levels (P less than 0.05). A bolus infusion of steroid hormone did not bring about any noticeable change in any of the levels of PGF2 alpha, TxB2, PGE2 or 6K. It may be inferred from these findings that PGs are synthesized in tumor tissue itself and released into plasma. Also, the finding that the levels of peripheral plasma PGs increased during chemotherapy suggested that such an increase in PG release could be one of the factors causing gastrointestinal toxicity. Based on the fact that there were no changes in levels of peripheral plasma PGs due to the administration of steroid hormone, however, we failed to support the proposal that steroid hormone suppresses the release of PG.