This study was designed to investigate the effect of chloroquine (CQ) on the release of preformed and generated mediators from rat serosal mast cells ( > 90% pure). CQ alone (10–1,000 μg/ml) induced the release of preformed mediators, while stimulation with calcium ionophore A23187 of CQ-treated cells resulted in suppressed releases of both preformed and generated (particularly prostaglandin D2) mediators. These effects were calcium-independent and reversible. Whether these results partly explain the efficacy of CQ in the treatment of mast cell-mediated dermatoses remains to be further investigated.