This study brings forth evidence in support of the existence of a direct relationship between antimitotic activity and the alteration of some enzymatic systems (NADH and NADPH tetrazolium reductase, LDH, SDH, MDH, IDH and cis-aconitase) which is produced by the administration of the methylhydrazine derivative Natulan® in both Ehrlich ascites carcinoma and N-13 ascites hepatoma. The greatest level of antimitotic activity occurred 8 h after the administration of 300 mg/kg of the preparation and was accompanied by a decrease in the activity of the systems of enzymes studied in the Ehrlich ascites carcinoma. This decrease could be observed by the application of histochemical methods. Twenty four hours after the experiment had begun, total metabolic and divisional recuperation had taken place in the Ehrlich ascites carcinoma. At the same time, there was intense reactivation of the enzymes examined in N-13 ascites hepatoma. In addition, this tumor presented a pronounced retention of glycogen