A general variation method is developed for determining: ( a ) the magnetic shielding of nuclear spin-spin interactions in molecules; ( b ) the electron-coupled nuclear spin-spin interactions in molecules. Using simple valence-bond and molecular-orbital wave functions calculations have been made for the HD molecule. It is concluded that effect ( a ) is small compared with ( b ). Both wave functions lead to values for the zero magnetic field splitting of about 49 c/s, in good agreement with the observed value of 43 c/s.