4‐Aminoimidazole‐5‐carboxamide, a component of human urine derived from the de novo purine biosynthetic pathway, was evidenced to undergo in vivo diazotization in rats following its sequential administration with NaNO2. The diazotization product, 4‐diazoimidazole‐5‐carboxamide, undergoes intramolecular cyclization to yield 2‐azahypoxanthine, the urinary presence of which was confirmed mass spectrometrically. 4‐Diazoimidazole‐5‐carboxamide demonstrated dose‐related mutagenicity in Salmonella typhimurium TA 100 and represents a potent electrophilic reactant similar to the proposed ultimate carcinogenic forms of arylalkylnitrosamines and arylnitrosamides. It is suggested that aryl and heterocyclic diazo compounds, as a class, warrant further study as environmental electrophiles representing potential biological hazard.