  {We aim to demonstrate the effect of atmospheric inhomogeneities on the
emergent specific intensity and radiation flux of a spectral line radiation.}
{We self-consistently solve the NLTE problem for a two-level atom in a 3D
atmosphere using the Cartesian grid. For that purpose, we use the 3D radiative
transfer code PORTA. By examining simple examples, we study cases where the
temperature inhomogeneities in the atmosphere models lead to the modification
of the emergent radiation.} {We show that specific temperature inhomogeneities
in the model atmospheres influence the emerging radiation, and that
interpretation of the stellar spectra based on a plane-parallel atmosphere
models can lead to erroneous conclusions about the atmospheric structure.}
