Introduction of the anticancer drug dacarbazine is the result of an attempt to design antagonists of purine biosynthesis. The mechanism of action of dacarbazine depends mainly on enzymatic transformation into as yet unknown reactive (electrophilic) intermediates. Recent studies have led to the identification and synthesis of 5-(3-hydroxymethyl-3-methyl)imidazole-4-carboxamide (HMTIC), a carbinolamine metabolite with methylating capacity. Although dacarbazine and related cytostatic triazenes are effective in the treatment of malignant melanoma and other human malignancies, dacarbazine has been demonstrated to be a carcinogen in laboratory rodents. Chronic administration of dacarbazine to rats of each sex induced predominantly thymic lymphosarcomas and mammary adenocarcinomas that were transplantable. Intraperitoneally injected 5-(3-methyl-1-triazeno)imidazole-4-carboxamide (MTIC), a metabolite of dacarbazine, induced a high incidence of mammary adenofibromas and a low incidence of uterine leiomyosarcomas. Animals treated with 5-diazoimidazole-4-carboxamide developed a low incidence of thymic, stomach, bladder or mammary tumours. Animals receiving 5-aminoimidazole-4-carboxamide developed a variety of tumours. No secondary malignancy has been reported in humans after treatment with dacarbazine alone. Despite their adverse effects, dacarbazine and related cytostatic triazene derivatives are useful clinically since their haematological toxicity is relatively moderate. As a rule, they are not cross-resistant with nitrogen mustard alkylating agents. It is hoped that research and development of second-generation N-(1-hydroxyalkyl)triazene compounds will lead to improvements in their clinical efficacy.