Young adult BD-IX rats made leukemic by intravenous infusion of 2 times 10(7) nucleated blood cells from leukemic donors (L 5222), showed lesions in the central nervous system 6 days after transfer when their leukocyte count was between 250,000 and 350,000 cells/mul. The brain was affected in only half of the animals, whereas all showed an infiltration of the meanings. Furthermore, all animals had extensive lesions in the spinal cord. The lesions consisted of nodules of leukemic cells and hemorrhages which were predominant in the lower half of the spinal cord and more abundant in the white than in the gray matter. Paraplegic appeared in some animals shortly before death. Since there is a consistent involvement of the meninges and spinal cord, the L 5222 leukemia seems to be a useful model for the study of leukemic infiltration of the central nervous system and the reaction to chemotherapy.