Carbon-14-labeled polyhexylcyanoacrylate nanoparticles, with diameters between 200 and 300 nm, were injected intravenously into nude mice. The distribution in liver, spleen, lung, heart, kidney, GI tract, gonads, brain, muscle, and serum was investigated by liquid scintillation counting. After a single injection, the radiolabel was cleared slowly with 45% remaining after 28 d and 8% remaining after 140 d. After repeated injection with an interval of 28 d (twice or thrice), relatively higher proportions of the dose were found in the spleen and lung as compared with those in other organs. No histological alterations were observed in the liver, spleen, or lung.