We have developed an experimental model of leptomeningeal tumor by inoculating Walker 256 carcinosarcoma cells into the cisterna magna of rats. This model was considered to be useful in studying pathophysiology and treatment of malignant brain tumors. In this study, the growth kinetics of this experimental tumor was investigated by using the immunohistochemical technique with an anti-BrdU monoclonal antibody. Walker 256 carcinosarcoma was subcutaneously passaged in female Wistar rats. Seven days after subcutaneous inoculation, the tumor was aseptically removed and minced in Hank's medium by scissors to make single cell suspension of the tumor. The cell suspension was adjusted to 1 x 10(5) cells/ml. And 0.1 ml was inoculated percutaneously into the cisterna magna of female Wistar rats weighing 150 gr. Every day after tumor inoculation, the animal (5 on each day) was sacrificed 30 minutes after intravenous BrdU (200 mg/kg) and perfused by saline. Then, the brain was removed, fixed in ethanol and embedded in paraffin. Coronal sections of the brain 6 mu in thickness were cut and stained by the indirect immunoperoxidase (ABC) method. The anti-BrdU monoclonal antibody (Becton-Dickinson) was diluted in 1:100. The sections were counterstained by hematoxylin. Labelling index (L.I.) of the tumor was obtained by counting immunoreactive cells under the microscope. L.I. of the subcutaneous tumor 7 days after inoculation was 52.4%. In the tumor 1 to 3 days after inoculation, L.I. was still low and between 11.9 and 15.1%. Four or 5 days after inoculation, the tumor cells grew in several layers in the subarachnoid space. L.I. at this stage of the tumor growth was 26.6 to 34.8%.(ABSTRACT TRUNCATED AT 250 WORDS)