The possible involvement of 5-fluorouracil (5FU) in the initiation process of the polymerization of n-butylcyanoacrylate monomer during nanoparticle formation was investigated. 5FU in acidic solution (pH 2-3) may interfere in the initiation process through its amino groups via the formation of zwitterions. The proposed zwitterionic mechanism of initiation was supported by the molecular weight profiles of the polymer, determined by gel permeation chromatography, and the covalent linkage of the cytostatic to the main polymer chain. H NMR analyses clearly demonstrated that a significant fraction of 5FU was covalently bonded to the poly(butylcyanoacrylate) chains through its amino groups preferentially through one of the two nitrogen atoms. In vitro release study performed shows that the investigated 5FU-loaded PBCN are suitable for sustaining delivery of 5FU.