Photodynamic therapy is an experimental treatment of superficial bladder tumors. Photofrin, a mixture of porphyrins, is the only photosensitizer in clinical use in the U.S.A. and its major side effect is prolonged cutaneous phototoxicity. In order to circumvent this problem of phototoxicity, new photosensitizers are being examined. Cutaneous phototoxicity may also be minimized by local administration of photosensitizer. Therefore, in this study, we investigated the photosensitizer chloro-aluminum sulfonated phthalocyanine (CASPc) in vivo in a rat bladder carcinoma model, and compared two different routes of CASPc administration. AY-27 rat bladder carcinoma cells were transplanted into rat bladders. Eight days after tumor transplantation the biodistribution of CASPc in bladder, skin, muscle and bladder tumor was determined by fluorescence measurements after dye extraction. Photosensitizer administered by intravenous injection and intravesical instillation, were compared. The concentration of CASPc in bladder and bladder tumor after intravenous injection and intravesical instillation was similar. The ratio of dye uptake between tumor and normal bladder after either administration was approximately two. Although no systemic absorption of the photosensitizer was observed after intravesical instillation, there was no reduction in tumor uptake or in the ratio between tumor to normal surrounding tissue. Therefore, no systemic side effects of skin phototoxicity are expected upon intravesical instillation. The microscopic biodistribution of CASPc after intravenous injection and intravesical instillation was also compared. After intravenous injection, the photosensitizer was distributed within the whole tumor with increased fluorescence around the microvasculature. In the normal bladder wall, weak fluorescence was seen in the area of the vasculature in the submucosa and the muscularis. After intravesical instillation, strong fluorescence was detected only at the tumor surface and in normal urothelium; no fluorescence was found in other areas of the tumor or in submucosa or muscularis. A comparison of the photodynamic treatment of model bladder tumors showed that tumor destruction after either method was similar but that there were less side effects to normal bladder wall after intravesical instillation of the CASPc. Intravesical administration of photosensitizers may, therefore, be a viable alternative to intravenous injection with potential for reduced systemic and normal tissue toxicity.