Groups of 10 male and 20 female B6C3F1 mice received 0, 500, or 1000 mg/kg/day 2'3'-dideoxycytidine (ddC) by gavage for 13 weeks. At the end of the 13-week exposure period all males and 10 females per group were necropsied while the remaining females were held for 1 month without further treatment. Thymic atrophy was present at the 13-week necropsy in male and female mice administered 1000 mg/kg/day and in females administered 500 mg/kg/day, but was not present in females following 1 month of recovery. Thymic lymphoma was present in 1 female that received 500 mg/kg/day and 1 female that received 1000 mg/kg/day. In a follow-up study groups of 70 female mice received 0, 500, or 1000 mg/kg/day for 13 weeks. At the end of the 13-week exposure period 20 mice per group were necropsied and the remaining animals held for 3 months without further treatment. Thymic atrophy was observed in ddC-exposed groups at the 13-week necropsy but not in mice allowed to recover for 13 weeks. Thymic lymphoma occurred in 3/50 mice that received 500 mg/kg/day and in 17/50 mice that received 1000 mg/kg/day but did not occur in mice from the vehicle control group.