14N nuclear magnetic resonance (NMR) measurements have been carried out for three members of 4'-n-alkoxy-3'-nitrobiphenyl-4-carboxylic acids (ANBC-n, where the number of carbon atoms in the alkoxy group, n, is 14, 16, and 22) in the temperature range 400-500 K. ANBC-16 and-22 show an optically isotropic D phase. The 14N NMR spectrum of the D phase showed a single peak, which may result from isotropically averaged quadrupole interactions around the 14N nucleus. Relaxation time measurements indicate the existence of two relaxational processes, faster anisotropic and slower isotropic motions, and suggest that in both cases ANBC molecules act as a dimer. The present 14N NMR results may be interpreted in the framework of the IPJR model, indicating that the structure of the D phase is a three-dimensional network continuous over the unit lattice.