The porphyrin derivative Photofrin-Il (PH-Il) is associated with positive photodynarnic therapy (PDT) responses (1,5) . However several properties of this photosensitizer, such as minimal absorption above 600 nm, chemical impurity and prolonged in-vivo retention, are not optimal. New photosensitizers are being synthesized and evaluated in an attempt to improve the efficacy of PDT (6) . Compounds with increased absorption further in the near infrared will improve photon utilization and depth of light penetration within tissue (7) . Derivatives of porphyrins, chlorins and phthalocyanines are among the compounds being examined (8, 9) . One second generation compound, mono-l--aspartyl chiorin e6 (NPe6), is an hydrophilic chlorin which exhibits promising in-vitro and in-vivo photosensitizing properties (8, 10-14) . N2e6 is chemically pure and has significant absorption at 664 nm. The photosensitizer appears to localize in the lysosomal compartment in cells and has in-vivo tissue distribution properties similar to PH-Il (12,14) . NPe6 mediated PDT is effective against a mouse mammary carcinoma when short time intervals (4-6 hours) between drug injection and light exposure are used (8) . Clearance of N2e6 from blood accounts for the minimal tumor and skin photosensitivity when a 24 hr interval between drug administration and light treatment is used. In the current study, we evaluated in-vivo metabolic properties of NPe6 and quantified PDT responsiveness as a function of the time interval between drug administration and light treatment.