In this work we describe an experimental model to evaluate the photodynamic toxicity on amphibian embryos, as well as the protective effect of antioxidants against the lethal oxidative stress induced by photosensitization. Bufo arenarum embryos were treated with 10mg/l methylene blue (MB) in AMPHITOX solution for 72h and then irradiated with a red laser or white light for variable times. Both light sources affected the survival of MB-treated animals and lethal effects occurred within the initial 12h post-irradiation. For white light irradiation, the most effective phototoxic condition in our study, the LD10, 50 and 90 at 6h post-irradiation corresponded to 13.57, 19.87 and 29.10J/cm 2 , respectively. To explore the action of antioxidants against the photogenerated oxidative stress, MB-treated embryos were incubated with 1mM glutathione (GSH) or ascorbic acid (AA) during 48h before irradiation. For GSH and 21.6J/cm 2 irradiation, the survival increased from 20 to 90%, whereas 100% survival was achieved with AA even after 43.2J/cm 2 irradiation. These results indicate that both the lethal photodynamic effect and its prevention by antioxidants can be evaluated by means of a simple toxicity test employing amphibian embryos.