The pharmacokinetics of the photosensitizer 5,10,15,20-tetra(m-hydroxyphenyl)chlorin (mTHPC) was investigated in the plasma of 20 patients by absorption and fluorescence spectroscopy. The temporal behavior was characterized by a rapid decrease in concn. during the first minutes after i.v. injection of 0.15 mg/kg mTHPC. A min. concn. in the plasma was reached after about 45 min. The drug concn. then increased again, attaining a max. after about 10 h, after which it decreased again with a half life of about 30 h. Irradn. tests in the oral cavity at different time intervals after the injection revealed that the tissue reaction was only partially correlated with the mTHPC plasma level. The tissue response was stronger at later drug-light intervals (1-4 days) than during the first hours after injection even though the mTHPC plasma concn. was higher at the shorter times. Relative mTHPC concns. were also measured in the mucosae of the oral cavity, the esophagus and the bronchi of 27 patients by light-induced fluorescence spectroscopy using an optical fiber-based spectrometer. These measurements were performed prior to photodynamic therapy (PDT), 4 days after injection of the photosensitizer. Highly significant linear correlations were found between the relative mTHPC concns. in the mucosae of these three organs. Likewise, the plasma levels of mTHPC measured just before PDT were significantly correlated with the mTHPC concns. in the three types of mucosae mentioned above. These results indicate that mTHPC plasma levels measured just before PDT can be used for PDT light dosimetry.