In this communication we discuss the in vivo parameters such tissue distribution, body clearance and toxicity following administration of carbon nanotubes (CNT). For this propose, therapeutically-relevant doses of serum protein-coated purified CNT (pCNT) and functionalized CNT (f-CNT) were administered intravenously in rodent species[1-3]. Striking differences were found between the liver and lung tissues of animals injected with nanotubes. pCNT exhibited significant accumulation in both tissues 24 h post-administration. f-CNT circulated in systemic blood circulation for 5 min, accumulated in the renal cortex and rapidly trespassed the glomerular filtering system and excreted in urine in the absence of any tissue accumulation or damage. The biocompatibility and biodistribution of CNT appears to be dramatically dependent on the degree of surface functionalization of these nanostructures, making f- CNT viable material for a variety of biomedical applications.