Prostaglandins (PGs) are considered to play important roles as contraction regulating factors in the mechanism of human uterine contraction. On the other hand, steroid hormones released in large amounts in late pregnancy appear to be closely related to the maintenance of pregnancy, fetal growth, and onset of delivery. To clarify the interactions between PGs and steroid hormones, we studied PGE2 and 6-keto-PGF1 alpha production on human myometrial cell culture. Cultures of myometrial cells from premenopausal patients with benign uterine diseases were established, and PG production in serum-free media and in the presence of various concentrations of progesterone (P), cortisol (F) and dehydroepiandrosterone-sulfate (DHAS) was studied. The following results were obtained: 1) The PGE2 and 6-keto-PGF1 alpha concentrations were 1,410.4pg/ml and 600pg/ml at 10 minutes after changing the medium, decreased transiently at 30-60 minutes, and increased thereafter. 2) When the cells were cultured with various concentration of P for 24 hours, PGE2 production was suppressed in the presence of P at 10(-7)M or above, but 6-keto-PGF1 alpha production showed a significant dose-dependent increase. 3) In the presence of F, PGF2 production shows a dose-dependent increase at high doses, but 6-keto-PGF1 alpha production markedly decreases at 10(-6)M or above. 4) In the presence of DHAS, PGE2 production increased markedly at 10(-6)M or above. These findings indicate that steroid hormones are regulatory factors in the production of PG by the myometrium.