A first-principles pseudopotential method has been used to study some potentially important metastable defects in N-doped GaAs. Formation energies have been obtained and related to those of As and Ga vacancies in the intrinsic material. Of the structures considered, two are identified that crucially depend on overall material stoichiometry. These are N on a Ga site (NGa) or a NN dimer on an As site (NNAs). NGa has a partly shallow energy-level structure with a fully occupied level lying near the valence-band edge, whereas NNAs has localized character with a midgap deep level. Both defects may be responsible for the onset of a reduction in the apparent band gap that has been recently observed in heavily N-doped GaAs prior to alloying.