The imidazole structure is involved in the formation of several compounds in the human metabolism including methyl imidazole, imidazole acetic acid, histidine, histamine, carnosine and homocarnosine. A number of these compounds are widely distributed metabolites such as histamine which is present in basophil and mast cells as well as being present in muscle and brain cells. This work advances detailed proposals on the metabolic chemical reactions which produce and decompose the compounds. The activity of these compounds in detrimental allergic medical conditions is discussed and the origin of the medical observations of the imidazole based medical compound known as cimetidine is discussed. (c) 2011 The Canadian Society of Clinical Chemists. Published by Elsevier Inc. All rights reserved.