We have studied the efficacy of intrathecal ACNU against experimental leptomeningeal tumors. In the present report, the effect of ACNU on the growth kinetics of the tumor was evaluated by the immunohistochemical technique using anti-BrdU monoclonal antibody. The experimental leptomeningeal tumor was developed by inoculation of Walker 256 carcinosarcoma cells into the cisterna magna of rats. Seven days after the inoculation of tumor cells, the animals were treated either by intravenous (15 mg/kg) or intrathecal (1.5 mg/kg) ACNU. Four, 12, 24, 48, 96 or 144 hours after treatment, the animals received intravenous BrdU (200 mg/kg). Thirty minutes thereafter, they were sacrificed and the brain was removed. L. I. was calculated by counting the immunoreactive tumor cells. L. I. of the tumor without treatment on the seventh day after inoculation was over 40%. L. I. began to decrease 24 hours after intravenous ACNU, and remained 11% up to 96 hours. On the other hand, L. I. already decreased to 20% 4 hours after intrathecal ACNU and remained to be low (17%) up to 48 hours. However, L. I. increased to 38% at 96 hours. Thus, the effect on the growth kinetics of the tumor differs between intravenous and intrathecal ACNU. These results are considered to be useful informations for determining the optimal dosage of the antineoplastic agent against the brain tumor and developing the effective combination chemotherapy.