Intracerebral inoculation of Murine Sarcoma Virus (MSV) into rats induces highly vascular brain tumors. Blood flow, F, and a unidirectional blood-to-tissue transfer constant, K, were measured concurrently in 16 separate brain tumors in 13 rats. Regional measurements of F and K were obtained using double-label quantitative autoradiography with 131I-iodoantipyrine, 14C-alpha aminoisobutyric acid, and computerized microdensitometry with image analysis. Regional tissue extraction fractions (E) were calculated for different brain and tumor regions. The following observations were made: (1) intracerebral inoculation of MSV in rats of different ages produced a single histologic tumor type (angiosarcoma), although tumor yield decreased and latency increased when older rats were inoculated; (2) whole tumor blood flow values averaged 63.9 ± 20.9 (SD) ml hg−1 min−1, less than F of tumor-free gray matter; (3) the mean value of the transfer constant was 1.19 ± 0.84 ml hg−1 min−1, 5 times the value in contralateral tumor-free brain; (4) moderate regional variation of F and K values were observed within each tumor, with increased K values extending into the regions of brain around tumor; (5) mean extraction fractions (E) of AIB for whole tumor were low (range = 0.0021 to 0.08), with a distinct and decreasing gradient from tumor center to BAT; (6) there was poor correlation between F, K and E values, and tumor histology, including regions that microscopically appeared highly vascular; (7) in most regions of the MSV-induced brain tumors, capillary permeability and/ or surface area seem to be determinant in blood-to-tissue transport processes.