It is still controversial whether edema fluid around a malignant brain tumor is derived from capillaries inside the tumor itself or not only from tumor vessels but also from peritumoral tissue. The purpose of this study is to clarify the region where the capillary permeability is increased using double autoradiographic method. A suspension of 1 X 10(4) rat glioma cells (RG-12) was stereotactically implanted into the right basal ganglia of C-D Fisher rats. In this model, all animals are dead 20 +/- 1 days after the implantation. Two kinds of tracers, 14C-alpha-aminoisobutyric acid (AIB) and 131I-human serum albumin (HSA), were administered 14 to 17 days after the tumor implantation. In all rats, 14C-AIB was intravenously injected 10 min before decapitation. 131I-HSA was given 10 min before decapitation in five animals (group 1), one hour before in four animals (group 2), and six hours before in three animals (group 3). Autoradiograms for 131I-HSA were exposed for initial one to two days after the decapitation, while the exposure for 14C-AIB was delayed until 4 months later. Autoradiograms of these two tracers were compared with the corresponding sections with H-E stain for neuropathological examination. In group 1, the distribution of HSA as well as AIB was quite similar to tumor itself. In group 2 and group 3, the distributions of AIB were similar to the tumor. However, the distributions of HSA were 1.4 fold larger (group 2) and 3.6 fold larger (group 3) than the tumor and expanded into the peritumoral region.(ABSTRACT TRUNCATED AT 250 WORDS)