Mast cells are immune effector cells that are involved in allergies and inflammation through the release of mediators such as histamine, PGs, and cytokines. Uncoupling protein 2 (UCP2) is a mitochondrial protein that inhibits insulin secretion from beta cells, possibly through down-regulation of reactive oxygen species production. We hypothesized that UCP2 could also regulate mast cell activation. In this study, we show that mouse bone marrow mast cells (BMMCs) and human leukemic LAD2 mast cells express UCP2. BMMCs from Ucp2(-/-) mice exhibited greater histamine release, whereas overexpression of UCP2 in LAD2 cells reduced histamine release after both allergic and nonallergic triggers. Ucp2(-/-) BMMCs also had elevated histamine content and histidine decarboxylase expression. Histamine content was reduced by overexpression of UCP2 or treatment with the mitochondrial-targeted superoxide dismutase-mimetic (TBAP) tetrakis(4-benzoic acid) porphyrin manganese(III). Furthermore, Ucp2(-/-) BMMCs also had greater production of both IL-6 and PGD(2) as well as ERK phosphorylation, which is known to regulate PG synthesis. Intradermal administration of substance P, an activator of skin mast cells, and challenge with DNP-human serum albumin after passive sensitization induced significantly greater vascular permeability in the skin of Ucp2(-/-) mice in vivo. Our results suggest that UCP2 can regulate mast cell activation.