Purpose: The present study was initiated to det. the conditions under which a single photodynamic treatment would induce maximal damage to a tumor with no or at least minimal reversible damage to a normal striated muscle. Methods and Materials: The technique of interstitial light delivery was used after prior 0.5 mg/kg tetra(m-hydroxyphenyl)chlorin administration in a hamster model. After having estd. the threshold light doses required for minimal muscle damage, the same light doses were applied to squamous cell carcinomas to evaluate the efficiency of interstitial photodynamic therapy. Sixteen and 96 h after the injection, irradn. at 650 nm was performed on the thigh muscle of the left hind leg. The applied light doses ranged between 0.3-15 J and were delivered at an intensity of 44 mW per cm of diffuser length. Results: The threshold of muscle damage was obtained using light doses of 1.5-3 J at two drug-light intervals of 16 and 96 h, resp. More than 85% of the tumor mass was destroyed when lesions were illuminated using these threshold conditions. In terms of immediate short-term tumor response, this means that for the given irradn. conditions, a relatively low threshold energy of only 1.5 or 3 J, depending on the drug-light interval, is sufficient to induce massive tumor destruction with minimal muscle damage. Conclusion: These results have implications for evaluating interstitial PDT for squamous cell cancers in unfavorable localization in the oral cavity or pharynx, such as at the base of the tongue.