The large strain uniaxial tensile deformation behavior of nylon 6 plasticized by deuterium oxide is investigated by an in situ stretching device built in a deuterium nuclear magnetic resonance (NMR) probe. The D2O molecules probe the environment of the amorphous regions in nylon 6; they do not exist in a "free" state and remain associated with the amide groups by hydrogen bonding during deformation. Deuterium NMR spectra show that the quadrupolar splitting varies linearly with strain throughout the experiment, indicating that the orientation of the D2O molecules in the amorphous regions is simply a function of strain and not of stress. The line width increases rapidly with strain at low to moderate strains but more slowly at large strains (in the strain-hardening regime). From low to moderate strains, larger changes in line width arise from a decrease in the translational motion of the D2O molecules between amide groups in the amorphous chains during elastic deformation and during the transformation of the lamellar structure of nylon 6 to a fibrillar one. At large strains, the existence and deformation of the fibrillar structure cause a slower decrease in the translational motion of the plasticizer.