5-aminolevulinic acid (5-ALA) is an interesting photosensitizing substance for photodynamic therapy (PDT), successfully applied topically for urological malignancy. In gastroenetrology it has proven efficacy for treatment of some GI neoplasms after systemic administration. This study was aimed at investigating the possibility of topical 5-ALA administration also for the PDT of gut cancer in a mice model. 5-ALA solution at different concentrations (5%, 1.5%, and 0.5%) was instilled in the colon of mice, which was later removed and examined by fluorescence microscopy. The results of fluorescence studies were compared with those obtained in a control group treated with 5-ALA given systemically. Satisfactory epithelial fluorescence levels and good selectivity between gut layers was obtained after intracolonic 5-ALA instillation. However, mean fluorescence intensity was higher after systemic drug application. Our results suggest that 5-ALA may probably be used topically for the PDT of some gut neoplasms. © 1999 Society of Photo-Optical Instrumentation Engineers.