Several parameters affect clin. trials in photodynamic therapy and influence the therapeutic outcome. Beside drug dose, light dose, drug-light interval and other variables, the fluence rate is a parameter that can influence the therapeutic results. In this study we have evaluated the fluence rate effect with a second-generation photosensitizer, tetra(m-hydroxyphenyl)chlorin (mTHPC) using a 7,12-dimethylbenz(a)anthracene-induced early squamous cell carcinoma of the Syrian hamster cheek pouch as a tumor model. Following injection of 0.5 mg/kg of mTHPC, irradn. tests were performed at two drug-light intervales, 4 and 8 days. Wavelength and light dose were adapted from those applied routinely in clin. trials. Irradiations at 652 nm were carried out with fluences ranging from 8 to 20 J/cm2 delivered at fluence rates of 15 and 150 mW/cm3. Similar tests were also performed at 514 nm with a fluence of 80 J/cm2 delivered at fluence rates ranging from 25 to 125 mW/cm2. At both wave-lengths and drug-light intervals for a given fluence. the higher fluence rates resulted in less tissue damage in tumor and healthy mucosae. However, the lower fluence rates yielded slightly less therapeutic selectivity. This study confirms that the fluence rate is of major importance in clin. PDT.