We have previously investigated the possibility of using hypericin as a diagnostic tool for the fluorescence detection of flat bladder carcinoma. In these clinical studies, it was shown that following intravesical application in humans, hypericin becomes selectively localized in transitional papillary carcinoma and carcinoma in situ (CIS). In the present study, we characterized the biodistribution of hypericin in rat bladder tumor and normal bladder layers after intravesical administration. The biodistribution was evaluated using fluorescence microscopy with computerized image analysis to image and quantify the fluorescence of hypericin across the urothelial tumor and normal bladder wall. The results show that the photosensitizer intravesical administration route provides selective labelling of both the tumor and normal urothelium. A hypericin dose and instillation time-dependent increase in the hypericin fluorescence intensity in both the tumor and urothelium was observed, without significant hypericin fluorescence in the submucosa and muscle layers. The highest fluorescence ratios in hypericin accumulation in the tumor and normal urothelium to the muscle layer were achieved at 4 h with 30 micro M hypericin (20:1 for the urothelium to muscle layer and 30:1 for the tumor to muscle layer). The difference in fluorescence intensity in tumor tissue to the muscle layer following instillation of 8 micro M hypericin was 11:1, 25:1 and 28:1 at 1, 2 and 4 h, respectively. The difference in fluorescence intensity in tumor tissue to the muscle layer using 30 micro M hypericin was 17:1, 27:1 and 31:1 at 1, 2 and 4 h, respectively. The highest absolute fluorescence levels were observed in the tumor at 4 h with 30 micro M hypericin instillation. The results suggest that under these conditions, PDT with hypericin is likely to produce uniform urothelial tumor eradication, without causing damage to the underlying muscle layers.