In this study of 25 central nervous system (CNS) tumor-bearing rats, the CNS biodistribution of intravenously administered, indium-labeled liposomes was investigated. In 16 animals, the blood-brain barrier and blood-tumor barrier were modified using intracarotid administration of etoposide. In control animals, analysis by autoradiography and well-counting experiments demonstrated uptake of liposomes in the tumor-bearing hemisphere (% injected dose/g tissue = 0.135) with minimal uptake in the non-tumor-bearing hemisphere (% injected doselg tissue = 0.007),p<0.01. Unilaterul intracurotid etoposide administration enhanced li-posome uptake in both hemispheres—0.215 and 0.023 (tumor-bearing and non-tumor-bearing), respectively. The presence of meningeal tumor involvement in non-tumor-implanted hemispheres increased liposomul uptake IO-fold. These findings may have clinical applicability in designing therapeutic protocols for the treatment of CNS tumors.