The understanding of tumor oxygenation at the microvascular level in an orthotopic model may provide usefir1 insight into tumor physiology, therapeutic response and development of protocols to study tumor behavior. In this paper the vascular status and the patho-physiolo~icalc hanges occuring during angiogenesis are studied in an orthotopic brain tumor model using a non-invasive multi-modality approach based on near inji-ared (NIR) dzffuse optical spectroscopy (DOS) along with magnetic resonance imaging (MRI). We report a direct correlation between tumor size and intratumoral microvessel density (MID) and tumor oxygenation. The relative decrease in the oxygen saturation value with tumor growth indicates that though blood vessels infzltrate and proliferate the tumor region, a hypoxic trend is clearly present.