This diploma thesis aimed for identifying the intestinal uptake of nanoparticulate systems related to the particle size. In particular, different endocytic mechanisms were studied, using a triple-cell culture model, which simulates the intestinal mucosa. The presented cell culture model meets all important physiological criteria in order to provide a reliable tool for intestinal uptake studies. For all experiments, plain polystyrene particles with different sizes (i.e., 50 nm, 100 nm and 200 nm) were used. These model particles, which are labelled with a red-fluorescent dye, were characterized with respect to their size, size distribution and zeta potential in different media. In addition, possible cytotoxic effects of the nanoparticles where investigated. To this end, the cell membrane integrity and the mitochondrial activity of the cells were assessed. To study cellular endocytic mechanisms, specific endocytic inhibitors were used, which selectively block clathrin-mediated endocytosis (CME) and/or caveolae-mediated endocytosis (CvME). Additionally, the cytotoxic potential of the used inhibitors chlorpromazine, genistein and dynasore in different concentrations were determined. Next, the cell culture model was treated with inhibitors, nanoparticles and specific endocytic markers, to study the nanoparticle uptake. Confocal laser scanning microscopy was used to evaluate the cellular uptake of the fluorescent particles. Furthermore, specific co-localisation coefficients were calculated in order to quantify the co-localisation of the particles with specific endocytic markers. The in vitro studies showed that both CME as well as CvME are involved in the cellular uptake of the investigated model-nanoparticles. The majority of nanoparticles was internalized via CME, however, with increasing particle size the CvME became more dominant.