To study the effects of photodynamic therapy on the tissues of the tongue, 20 New Zealand White rabbits were injected intravenously with haematoporphyrin derivative. Their tongues and those of 2 controls were illuminated with 100 J/cm2 of red light from a copper vapour-pumped dye laser emitting light at 625 nm. The rabbits were sacrificed at intervals between 2 days and 12 weeks following treatment. Their tongues were examined histologically. Initially there was marked damage to mucosa, muscle and blood vessels. However, this healed by a mixture of regeneration and scarring.