In order to clarify the mechanism by which oxytocin induces the generation of prostaglandin (PG) in human myometrial smooth muscle cell, we determined the concentration of PGE2 and 6ketoPGF1 alpha in the culture supernatant of human myometrial monolayer cells stimulated by oxytocin. PGE2 and 6ketoPGF1 alpha demonstrated a biphasic increasing curve when oxytocin was added. The first increase, phase 1, was a transient phenomenon with a peak at 20 seconds whereas the second increase, phase 2, was a continuous phenomenon starting at 120 seconds. Moreover, phase 1 was significantly inhibited by the DG lipase inhibitor, RHC80267. Phase 2 was significantly inhibited by the PLA2 inhibitor, Mepacrine. Phase 1 and 2 were dose dependent in the range 10(-10) to 10(-6) M. PG production at rest during which 6ketoPGF1 alpha was higher than PGE2, was reversed by oxytocin stimulation. These results suggest that oxytocin stimulation would actuate biphasic PG production, and this mechanism would probably regulate the myometrial construction in labor-pain initiation.