['This study was undertaken to test that uterine mast cell degranulation alters human myometrial contractility in vitro and to define what mediators are involved in this process.', 'Longitudinal myometrial strips prepared from biopsy specimen obtained from the lower uterine segment of women at preterm and term gestation (with and without labor) were studied. Contractile responses to compound 48/80, a mast cell degranulator, were compared in the absence or presence of a mast cell stabilizer, H 1 and H 2 receptor antagonists, cyclooxygenase, and lipoxygenase inhibitors.', 'Compound 48/80 increased myometrial contractility in all groups. The mast cell stabilizer cromolyn inhibited contractility, whereas the cyclooxygenase inhibitor ibuprofen, the H 1 -receptor antagonist S(+)-chlorpheniramine maleate, but not the H 2 antagonist cimetidine, only slightly attenuated this effect. The lipoxygenase inhibitor linoleyl hydroxamic acid augmented the responses to compound 48/80 in the preterm but not in the term group.'] Does degranulation of uterine mast cell modify contractility of isolated myometrium from pregnant women?