The apoptotic response of normal brain and intracranial VX2 tumor following photodynamic therapy mediated by five different photodynamic drugs, Photofrin, ALA, AlClPc, SnET2 and mTHPC, was evaluated in a preliminary retrospective analysis. Rabbit brain, with or without tumor, was treated by PDT with interstitial light delivery. Histological sections at 24 h post PDT were assessed by the TUNEL assay. Confocal fluorescence microscopy was used to determine the total apoptotic cell count and the spatial distribution of apoptotic bodies within the tissue. The data were confirmed qualitatively by light microscopy on adjacent H&E-stained sections. Light-only and drug-only controls produced background levels. The highest apoptotic count was seen with Photofrin. The counts in AlClPc-treated animals were not above the background level, while the other 3 photosensitizers gave intermediate levels. With some, but not all, drugs the spatial distribution of apoptotic bodies correlated well with the light fluence distribution. Apoptosis was seen outside the zone of frank coagulative necrosis. There was not apparent drug-dose dependency at the relatively high doses used here. The retrospective nature of this study did not allow optimization of the treatment parameters. Nevertheless, the findings have potentially significant implications, both for understanding the mechanisms of apoptosis in brain tissue and for improving the clinical use of PDT for treatment of patients with malignant brain tumors.