Regional blood-to-tissue transport of 14C-alpha-aminoisobutyric acid (AIB) was measured in avian sarcoma virus (ASV)-induced rat brain tumors and expressed as a unidirectional transfer rate constant (K). The magnitude of K was variable and did not correlate with histologic classification or specific features of the tumors, with the possible exception of sarcomas. Averaged mean K was highest for anaplastic astrocytomas and lowest for gemistocytic astrocytomas (GA); however, the range of measurement within the individual tumor classifications (except GA) was broad. K was not consistently related to tumor size or tumor location; choroid plexus and subependymal tumors were exceptions and had higher K values. The averaged mean K of tumor-free brain in the contralateral hemisphere (CBA) was comparable to that of control animals. The averaged mean K in brain adjacent to tumor was greater than in CBA and generally one-third to one-half of the value in tumor periphery, indicating that these tumors affect the permeability characteristics of adjacent brain capillaries. Estimates of the fractional extraction (E) of AIB by the tumors ranged between 0.009 and 0.2; E's in this range indicate that tumor capillaries are not freely permeable to this solute.