BACKGROUND We have previously shown that in sensitized guinea pigs premature labor can be induced by a type I hypersensitivity reaction. We further hypothesize that premature labor occurs due to increased uterine contractility caused by activation of mast cells and possibly eosinophils, and collective release of their mediators. The objective of this study was to test the hypothesis that IgE-independent mast cell degranulation could increase uterine contractility. METHODS Longitudinal uterine strips from nonpregnant and pregnant guinea pigs were incubated in organ chambers with vehicle, histamine H(1), serotonin 5-HT(2)/5-HT(1C), thromboxane A(2), leukotriene D(4) receptor antagonists, mast cell stabilizer, and cyclooxygenase or lipoxygenase inhibitors. Then, supernatant, obtained after activation of a mast cell line (MC/9) with compound 48/80, culture medium, or compound 48/80 alone were added. Cumulative concentration-response curves to histamine and serotonin were also obtained. RESULTS The supernatant and compound 48/80 significantly increased contractility of uterine strips. A mast cell stabilizer considerably reduced the effect of compound 48/80. Other substances attenuated uterine contractile responses to supernatant and compound 48/80, and responses varied depending on the pregnancy period. Histamine and serotonin increased contractility of uterine strips, and uterine sensitivity to these agents were dependent on gestational age. CONCLUSIONS In summary, mast cells increase uterine contractility through multiple mediators, and uterine responses to these mediators are dependent on gestational age. We postulate that the simultaneous release of these mast cell/eosinophil mediators in the uterus could be a stimulus to trigger and/or maintain myometrial contractions during preterm and term labor.