Photodynamic therapy utilizes the tumor localizing drug dihematoporphyrin ether and red laser light to produce both direct tumor cell destruction via damage to mitochondrial membranes, and also indirect cell kill via destruction of the tumor vasculature. As a first step towards examining the mechanistic relationship between metabolic and vascular effects of photodynamic therapy, murine RIF-1 tumors were treated with a subcurative treatment (500 J/cm2). Tumor metabolic status was monitored using in vivo 31P NMR before, during and after the treatment. The tumor blood flow immediately before and after treatment was measured by direct intratumor injection of D2O saline and observation of the tracer signal clearance from the tumor via 2H NMR. During the photodynamic therapy treatment, significant decreases were observed for the nucleoside triphosphate concentrations, tumor pH and tumor blood flow, while inorganic phosphate concentrations increased. Animals treated with laser light alone and those not given any treatment, demonstrated no significant changes in tumor metabolic status, tumor pH or tumor blood flow. Further studies are required to determine whether tumor blood flow or metabolic status is affected first.