Mast cells are involved in allergic reactions, where they secrete numerous vasoactive, inflammatory and nociceptive mediators in response to immunoglobulin E (IgE) and antigen. However, they have also been implicated in inflammatory conditions, such as painful bladder syndrome/interstitial cystitis (PBS/IC), irritable bowel syndrome (IBS) and migraines, all of which occur more often in women and are exacerbated during ovulation, but are suppressed during pregnancy. Mast cells express high affinity estrogen receptors and estradiol augments their secretion, while tamoxifen inhibits it. Here we report that progesterone (100 nM), but not the structurally related cholesterol, inhibits histamine secretion from purified rat peritoneal mast cells stimulated immunologically or by substance P (SP), an effect also documented by electron microscopy. These results suggest that mast cell secretion may be regulated by progesterone and may explain the reduced symptoms of certain inflammatory conditions during pregnancy.