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 PGF2α on myometrial strips obtained during active, spontaneous labor. Both PGE2 and PGF2α 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 PGF2α 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.