Photodynamic therapy (PDT) can be used to cause vascular collapse and blood flow stasis of the irradiated pathological neovascularisation appearing in several diseases such as age-related macular degeneration (AMD) and cancer. We hypothesized that this PDT-related interruption of vessel integrity may lead to an increased transvascular passage of drugs that could be used as a drug delivery pathway. Thus, preceding the occlusion of the pathological vasculature, PDT could be used for instance as a local drug delivery pathway to administrate an anti-angiogenic drug in the case of AMD or chemotherapy in the case of cancer, potentially improving combination therapies. In the case of chorioneovessels (CNV) due to AMD, the recurrence of the exudative AMD component of the weeks/months after PDT, due to the re-opening and/or re-growth of neovessels might be avoided by adding an anti-angiogenic factor such as anti-VEGF or anti-inflammatory drug before, during or shortly after PDT. In the case of cancer, the starvation of tumour cells induced by the PDT occlusion of blood vessels feeding the tumour might be combined with a chemotherapeutic agent for the direct kill of the cancer cells themselves. It has been reported that following the light application in PDT, a physiological cascade of responses on the one hand leads to vascular occlusion but may also induce a vascular permeability enhancement. The aim of this thesis is to find conditions where this increase in leakage due to PDT can be observed, to characterize it and to take advantage of this phenomenon to develop the basis of a novel combination therapy approach. Hence in this thesis, pre-clinical experiments were performed in the vasculature of the chorioallantoic membrane model (CAM) of the chicken embryo and in the dorsal skinfold optical chamber of nude mice observed by intravital microscopy (IVM). In the CAM, no PDT-induced leakage of a fluorescent dye (FITC-dextran) was observed unless an anti-aggregating factor, such as aspirin was added. In the chicken embryo model, delaying the blood clot appears to be an essential process to allow effective potential drug delivery. In the dorsal skinfold of the nude mouse, the inflammatory response after PDT was observed and quantified. This revealed that PDT induces a time dependent acute inflammatory response as shown by increased number of leukocytes "rolling" along the vessel wall after treatment. This was observed over a 2 hour period following PDT. The quantification of the microvascular leakage showed a continuous FITC-dextran leakage from the vasculature treated by PDT to the interstitial space. This local leakage was clearly increased by the inflammatory status of the tissue (observed by quantifying the rolling leukocytes and by histology). This concept has the potential to improve the drug delivery of anti-angiogenic drugs in the eyes of patients treated for AMD and could also be applied to improve the uptake of cytostatic drugs in tumours.