Besides toxicity tests, biokinetic studies are a fundamental part of investigations to evaluate a safe and sustainable use of nanoparticles. Today, gold nanoparticles (Au NPs) are known to be a versatile tool in different areas such as science, engineering or medicine. In this study, we investigated the biokinetics after intravenous and intratracheal applications of poly(ethylene glycol) (PEG) modified Au NPs compared to plain Au NPs. Radioactive-labeled Au NPs of 5 nm inorganic core diameter were applied to rats and the NP content in tissues, organs and excretion were quantified after 1-hour and 24-hours. After intravenous injection, a prolonged blood circulation time was determined for Au NPs with 10 kDa PEG chains. Non-PEGylated Au NPs and 750 Da PEG Au NPs accumulated mostly in liver and spleen. After intratracheal application the majority of all three types of applied NPs stayed in the lungs: the total translocation towards the circulation did not differ considerably after PEGylation of the Au NPs. However, a prolonged retention time in the circulation was detected for the small fraction of translocated 10 kDa PEG Au NPs, too. Copyright (c) 2010 Elsevier Ltd. All rights reserved.