Since 1953, quite a number of alkaloids containing the imidazole nucleus have been isolated that can be added to the originally small group of imidazole alkaloids. These new compounds are divided into Alkaloids Related to Pilocarpine and Other Imidazole Alkaloids. The last category comprises now over 80 alkaloids isolated from different sources. These imidazole alkaloids proved to be of strongly divergant structures, some of which have not yet been completely clarified. The relative and absolute stereochemistries of pilocarpine have been elucidated and some different syntheses of this medicinally important compound have been reported. However, the mechanism of many biochemical transformations still remains obscure. Relatively few simple derivatives of imidazole occur naturally; mainly products of fungi have been found, such as ergothioneine and hercynine, the betaine of histidine. This is rather astonishing because the amino acid histidine is a widespread constituent of many proteins. The base histamine is derived from histidine and plays an important role in allergic reactions in man and in animal tissues. © 1983, Academic Press, Inc. All rights reserved.