The novel bradykinin (BK) analog, RMP-7, was characterized in a series of in vitro tests to establish its selectivity as a B2 agonist. It was then used to study bradykinin's role in permeabilizing the blood brain barrier (BBB) and blood brain-tumor barrier (BTB), using an RG2 rat glioma model. These studies demonstrated that: (1) B2 receptor stimulation permeabilizes both the BBB and BTB in a dose-related fashion with greater effects observed in brain tumor-associated tissue, (2) the increased permeability is sensitive, rapid and transient, and (3) tachyphylaxis occurs with continuous agonist administration, suggesting autoregulation of the system's effects. These data therefore support the existence of a sophisticated, responsive and tightly regulated BK system whose activity modulates the permeability of the BBB.