Stability and structure of Na(NH3)(n) (n = 1-6) clusters as well as their ions have been investigated by the ab initio MO method and systematically compared with those of Na(H2O)(n) clusters. Both neutral and ion ammonia complexes with n greater than or equal to 4 have an interior structure where Na is surrounded by NH3 molecules rather than a surface structure preferred by Na(H2O)(n) clusters. Both neutral and ion clusters are stabilized by the Na-N bonds; the interior structure allows formation of a maximum number of such bonds in the first solvation shell. The calculated ionization potential as a function of the number of solvent molecules is in good quantitative agreement with experiment and behaves differently from that of the surface structures of Na(H2O)(n) clusters. In the interior structure the SOMO (singly occupied molecular orbital) electron is distributed on and between Na and solvent molecules inside the cluster; interior ionization is expected to take place.