The myometrial response to prostaglandins (PGs) before labor has been studied earlier in this laboratory by the use of a superfusion technique. In the present investigation the same technique was used to explore the contractile effect of PGE₂ and PGF_2Α on myometrial strips obtained during active, spontaneous labor. Both PGE₂ and PGF_2Α had a marked stimulatory effect on strips from the upper uterine segment. The lower segment reacted to PGE₂ by inhibition but not at all or slightly stimulatory to PGF_2Α Oxytocin constantly induced an excitatory response in both segments The results support the concept of a prostaglandin-mediated ‘fundal dominance’ during labor and indicate that PGs contribute to a relaxation of the lower uterine segment. Taken together with previously obtained data the present results provide evidence for a critical change in myometrial responsiveness to prostaglandins by the time of parturition.