The e.s.r. spectra of the negative ions of a variety of nitrogen heterocyclic molecules have been measured and the proton and nitrogen hyperfine coupling constants determined. The proton splittings are interpreted using Hückel-type molecular orbital calculations with an adjustable Coulomb parameter for the nitrogen atom. The results indicate that the nitrogen splitting is proportional to the unpaired electron density on the nitrogen atom, the proportionality constantQNhaving the value 25 ± 2 gauss. No significant dependence of the nitrogen splitting on the spin density on adjacent carbon atoms is observed.