We investigated differences in the oncogenic effects of methylnitrosourea (MNU) which were induced by varying dose schedules and changing administration routes. The nervous system represented the target organ when MNU was given intravenously to rats. Carcinogen by other routes resulted in decreased numbers of neurogenic tumors and the appearance of neoplasms at the injection site. Increased oral doses of MNU caused shorter survival times, a decreased incidence of neuroglial tumors, and increased numbers of thymic lymphomas and mesenchymal tumors of the nervous system. The results suggest that many tissues are susceptible to the oncogenic effects of MNU, but the degree of exposure necessary for neoplastic transformation varies.