A patient with cholinergic urticaria syndrome was found to have an unusually large number of muscarinic cholinergic receptors in the urticarial sites. The amplification of these receptors may account for her heightened sensitivity to acetylcholine discharge following exercise, heat, and emotion. The acetylcholine presumably acts directly on the mast-cell membrane to initiate histamine release and thereby punctate urticaria. In addition, an otherwise occult sensitivity to copper was detected with a 48 h contact patch test to the metal followed by an exercise challenge. Under these conditions, the patch test site developed hives after the autonomic stress stimulus of exercise but showed nothing on conventional patch testing before exercise. Such "cholinergic patch testing" is recommended for the demonstration of other examples of antigen enhancement of acetylcholine-mediated reactions.