In human parturition, progesterone withdrawal and estrogen activation are not mediated by changes in progesterone and estrogen levels. Instead, these events could be facilitated by changes in the responsiveness of the myometrium to progesterone and estrogens via changes in PR and ER expression. We hypothesized that functional progesterone withdrawal occurs by increased expression of the type A PR (PR-A), which suppresses progesterone responsiveness, and that functional estrogen activation occurs by increased myometrial expression of ERalpha and/or ERbeta. To test this hypothesis we compared the abundance of mRNAs (assessed by quantitative RT-PCR) encoding PR-A, PR-B, ERalpha, and ERbeta in nonlaboring (n = 12) and laboring (n = 12) term human myometrium. PR-A, PR-B, the PR-A/PR-B mRNA ratio, and ERalpha mRNA were significantly increased in laboring myometrium, whereas ERbeta mRNA was low and unchanged. The PR-A/PR-B mRNA ratio correlated positively with ERalpha mRNA levels in nonlaboring myometrium and with HOXA10 mRNA levels in laboring myometrium. Because progesterone inhibits ERalpha and HOXA10 expression, these findings indicate that myometrial progesterone responsiveness is inversely related to the extent of expression of PR-A relative to PR-B. ERalpha mRNA levels correlated positively with cyclooxygenase type 2 and oxytocin receptor mRNA levels in nonlaboring myometrium, indicating that the increase in ERalpha expression is directly associated with the activation of contraction-associated genes and estrogen responsiveness. These data indicate that in the term human myometrium, responsiveness to progesterone is controlled by the expression of PR-A relative to PR-B and that a significant increase in this ratio underlies functional progesterone withdrawal. Our data also indicate that functional estrogen activation occurs by increased expression of ERalpha and is linked to functional progesterone withdrawal. Interaction between the PR and ER systems in the human myometrium may be critical for the control of human parturition and the coordination of progesterone withdrawal and estrogen activation required for parturition.