The H -antagonists, metiamide and cimetidine were labelled 2 with deuterium and tritium in the 2-position of the imidazole ring by the uncatalyzed exchange reaction with deuterium and tritium oxide at 100°C. The sulfur-35 labelled metiamide was prepared by an exchange reaction with elemental sulfur-35 in refluxing pyridine and by the reaction of 2[(4-methyl-S-imidazolyl) methylthio]ethylamine with m e t h y l i s~t h i o c y a n a t e -~~S in refluxing ethanol.