Urticaria and angioedema may occur in skin and mucus membranes when mast cells are activated by various physical stimuli, including trauma, pressure, vibration, light, cold, heat, and (in rare cases) water. Experimental challenge of patients with cold-induced and cholinergic urticaria/angioedema in particular provides an in vivo model of mast cell activation in humans. This model synthesizes observations of the evolution of clinical manifestations, histologic analysis of tissue alterations, measurement of mediators released into the circulation, and assessment of leukocyte motility. The model in turn allows a characterization of mediators that exist preformed in mast cell granules or that are generated through interactions with other cell types. Release of these mediators produces a variety of biologic effects, including elaboration of certain enzymes and alterations in venular permeability, smooth muscle contraction, leukocyte motility, and the release of substances from other cell types.