One of the 'second generation' photosensitizing agents is N-acetyl chlorin e6 (NPe6). This product has a strong absorbance band at 665 nm, permitting treatment at a greater depth of tumor than earlier agents based on porphyrin structures. We examined the effects of fractionated drug administration on photodynamic efficacy. Prior studies had shown that it is the level of NPe6 in the circulation that predicts for photodynamic efficacy, indicating vascular shut-down to be the predominant mode of tumor control. Although pharmacokinetic studies revealed that >99% of NPe6 was lost from the circulation, it appears that a fractionated dosage protocol can promote photodamage to neoplastic tissue in vivo. This study also indicated the potential utility of an implantable micro array for tumor irradiation.