The NaI D lines at 5890/5896A are very prominent features in the spectrum of late-type stars. Nevertheless, little attention has been devoted to the potential use of those lines as chromospheric diagnostics. As a case study, we explore the dependence on chromospheric activity of the D lines in a star with T_eff = 3700K, log g = 4.7 and solar metallicity. The results are compared with the better studied hydrogen spectrum. We find that the D lines seem to be a promising diagnostic of the lower-middle chromosphere, that can complement the information given by lines like H alpha. We also find that, for detailed quantitative studies, it is necessary to include a proper treatment of the background opacities. Less important, instead, is the need of careful treatment of transitions induced by collisions with hydrogen atoms. Finally, our calculations make it clear that, for the most active stars, the level of coronal emission should also be taken into account.