AbstractSilica nanoparticles, radiolabeled with75Selenium were coated with 14 types ofω-functionalized surfactants covalently bound to the particle surface. The particles were suspended in phosphate buffered saline (PBS) and injected intravenously via the tail vein of Wistar rats. The animals were sacrificed after 5 different time points (30 min, 2 h, 6 h, 24 h, and 7 d), and two samples of each organ and two blood samples were weighed into vials. The radioactivity of each sample was measured in a LKB-Wallac CliniGamma counter. Coated silica nanoparticles accumulated in the liver at much lower levels than other colloidal drug carriers after short time periods (30 min). The liver accumulation increased after longer time periods due to a natural redistribution process. Surface modification by increasing the hydrophilicity and thickness of coating yielded higher and longer persisting concentrations in the intestine, blood, and the muscles. Initially increased lung concentrations were decreasing with time, propably due to migration of the alveolar phagocytes.