The toxic effects of 5-AZA-2'-deoxycytidine (5-AZA-CdR) administered as a 12 hr. i.v. continuous infusion to CD2F1 (Balb/c x DBA/2) mice were investigated. This i.v. route of administration of 5-AZA-CdR was chosen because it produced a potent antineoplastic effect in leukemic mice and would probably be the route used for clinical trials of this agent. The LD50 of 5-AZA-CdR for male and female CD2F1 mice was estimated to be 29.5 and 22.2 mg/kg, respectively. A toxic dose of 5-AA-CdR produced a leukopenia, thrombocytopenia, and weight loss in the mice. Most of these toxic effects were reversible with the exception of leukopenia which was still present 43 days after the infusion. Histopathological analysis of the mice during the acute toxic phase (day 7) and the recovery period (day 28) showed that 5-AZA-CdR at a dose close to the LD50 produced bone marrow hypoplasia, necrosis of the small intestinal mucosa and atrophy of thymus and testes. All these pathological lesions were reversible. The toxic effects produced by 5-AZA-CdR are consistent with its biological action, an agent that is cytotoxic to only proliferating cells.