The splitting of the tetrahedral Rb- and K-NMR lines in Rb3-xC60 and K3-xC60 is analyzed in terms of the mechanism of alkali-cation vacancies. The calculations indicate the presence of vacancies only in the tetrahedral positions, and the interatomic potentials between the alkali cations and the fullerene molecules account satisfactorily for the temperature dependence of the position of the distorted line.