 Plasma drug concentration measurements are an unreliable method to assess delivery of drugs across the BBB. In contrast, intracerebral microdialysis monitoring allows for approximate measurements within extracellular fluid ECF sampling of the brain. Microdialysis placement within the brain is not a novel technique and has been utilized routinely in the intensive care unit setting to measure brain metabolism by sampling of ECF of traumatic brain injury patients. Microdialysis catheters are now FDA approved and are being placed routinely with intracranial pressure monitors. This method allows for continuous measurement of ECF within a tumor or normal tissue. The dialysis probe has a semipermeable membrane which is less than 1 mm in diameter into which two sections of microcatheter are fused. Previous studies have demonstrated the feasibility of keeping the catheters in place of critically injured patients for up to 2 weeks. When placed at the time of surgical resection, the microcatheters are sterotactically implanted, placing the probe within the desired brain andor tumor region. Externally, the catheter is connected to a syringe pump, which delivers a low flow rate μlmin of continuous perfusion fluid Lactated Ringers or artificial CSF and dialysate is collected in a microvial from the outlet tube. This sterile, single use catheter is minimally invasive and developed to achieve optimal diffusing characteristics similar to passive diffusion of a capillary blood vessel. Just as in the function of brain capillary vessel, water, inorganic ions and small organic molecules freely diffuse across the membrane of the probe, whereas proteins and protein bound compounds are impermeable. Additionally, lipophilic compounds are poorly recovered. 