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The radioactivity level in blood, bile, urine and contents of parts of the gastro-intestinal tract in rats was studied after subcutaneous administration of 3-H-1,2-dimethylhydrazine (3-H-SDMH) which induces colonic tumours. The alkylation of DNA, RNA and protein in the intestinal mucosa, liver and kidneys was estimated 1 h to 28 days after 3-H-SDMH treatment from the 3-H-incorporation into these macromolecules. Administration of 3-H-1,2-diethylhydrazine (3-H-SDEH) which does not induce intestinal tumours was made as a control. Fifteen to 30 min after 3-H-SDMH treatment, marked radioactivity was found in blood, bile, urine and in contents of all regions of gastro-intestinal tract. After 3-H-SDMH administration no label occurred in the contents of localized segments of gastro-intestinal tract although it was present in blood, bile and urine. 3-H-SDMH methylated DNA, RNA and proteins of intestinal mucosa, liver and kidney to a high degree. One hour after 3-H-SDMH treatment the incorporation of label into protein of intestinal mucosa was higher than into liver and kidneys. 3-H-SDEH did not alkylate macromolecules in these organs but did so in thymus, spleen and brain, which are target organs for this carcinogen. After total hepatectomy, 3-H-SDMH did not methylate macromolecules of the intestinal mucosa. The following mechanism for the carcinogenic effect of SDMH is suggested. A carcinogenic metabolite of SDMH forms, in the liver, a conjugate with glucuronic acid. This glucuronide enters the gut both with bile and directly via the circulation. Microbial beta-glucuronidase releases the active metabolite which, in turn, alkylates tissue macromolecules.