In 13 Fischer 344 rats transplanted intracerebrally with 9 L gliosarcoma, 13 normal Fischer 344 rats and 4 clinical cases of malignant glioma, a new water-soluble nitrosourea (MCNU) was given and the concentration was measured in blood, tumor tissues, normal brains around the tumors and normal hemispheres by intravenous or intraarterial infusion of MCNU. At 5 min. after administration of MCNU 20 mg/kg (4-5 mg/body) in 9 L gliosarcoma bearing Fischer rats, mean MCNU concentration in the blood was not different between 20 micrograms/ml intravenous and 23 micrograms/ml intraarterial administrations whereas that in the tumor tissues by intracarotid infusion of MCNU was 40 +/- 14.4 micrograms/g which was about two times as much as 22.9 +/- 8.13 micrograms/g by intravenous infusion of MCNU. Mean MCNU concentration of normal brains around tumor tissues was 2.49 micrograms/g in intravenous and 8.95 micrograms/g in intracarotid infusion. MCNU concentration of tumor tissues in 4 cases of malignant gliomas was higher by intracarotid administration than by intravenous administration compared to that in the blood. Maximum tumor/blood ratio of MCNU was 1.94 in intracarotid administration for the malignant glioma. It is suggested that intraarterial administration was more useful than intravenous infusion as an administration route for malignant brain tumors.