The question of efficiency of radiative energy losses in spectral lines is addressed. In a semi-infinite atmosphere with constant temperature, the total radiative energy loss (integrated over all depths) in a spectral line without continuum is infinite, in both LTE and non-LTE. Thus, only local energy balances may be considered with such models. The authors give radiative cooling functions for various non-LTE parameters and structural lengths of a two-dimensional stratified atmosphere. At the surface, cooling is less efficient in non-LTE than in LTE. At large optical depths, both become equal and are non-negligible. In these layers horizontal transfer effects become important for the energy balance.