Determining the in vivo pharmacological profiles of carbon nanotubes (CNTs) is essential for the promising biomedical applications of CNTs, such as drug delivery. Using iodine-131 tracing we studied the fundamental behavior of hydroxylated single-walled CNTs (SWNTols) shortly after they were introduced into the animal body (from 2 min to 1 h) by providing the biodistribution data and pharmacokinetic parameters. The distribution was slightly influenced by injection modes, but in any mode radioactivity was found all over the body within 2 min except brain. Liver, kidneys, stomach and lungs are the target organs with high uptake of SWNTols. The SWNTols content in several tissues, such as heart, lungs, and muscle is positively correlated with its content in the blood, showing clearly that the blood stream brings SWNTols to the whole body. This work presents the initial in vivo behavior of the water-soluble functionalized SWNTs, providing also the basic data to show opportunities and limitations for realization of the CNT-based drug vehicle.