The influence of introducing heteroatom into carbon-based pi-conjugation on the intramolecular x-spin density distribution was examined for three kinds of nitronyl nitroxide radical molecules. The magnitudes and relative signs of the pi-spin densities for a heterocyclic substituted nitronyl nitroxide radical, 3-quinolyl nitronyl nitroxide were determined by liquid-phase (1)-H-ENDOR and TRIPLE spectroscopy. A pronounced negative spin density (p = -0.032) was found in the 2- or 4-position of the quinolyl ring. The p value is about 3/2 times as large as that of the non-heterocyclic molecules, phenyl nitronyl nitroxide and 1-naphtyl nitronyl nitroxide. The mechanism for the enhanced amplitude of the negative spin polarization in the quinolyl derivative is explained in terms of the topological relation between the heteroatom and the nitronylnitroxide group.