We developed a simple chemical method to obtain bulk quantities of N-doped, reduced graphene oxide (GO) sheets through thermal annealing of GO in ammonia. X-ray photoelectron spectroscopy (XPS) study of GO sheets annealed at various reaction temperatures reveals that N-doping occurs at a temperature as low as 300 degrees C, while the highest doping level of approximately 5% N is achieved at 500 degrees C. N-doping is accompanied by the reduction of GO with decreases in oxygen levels from approximately 28% in as-made GO down to approximately 2% in 1100 degrees C NH(3) reacted GO. XPS analysis of the N binding configurations of doped GO finds pyridinic N in the doped samples, with increased quaternary N (N that replaced the carbon atoms in the graphene plane) in GO annealed at higher temperatures (> or = 900 degrees C). Oxygen groups in GO were found responsible for reactions with NH(3) and C-N bond formation. Prereduced GO with fewer oxygen groups by thermal annealing in H(2) exhibits greatly reduced reactivity with NH(3) and a lower N-doping level. Electrical measurements of individual GO sheet devices demonstrate that GO annealed in NH(3) exhibits higher conductivity than those annealed in H(2), suggesting more effective reduction of GO by annealing in NH(3) than in H(2), consistent with XPS data. The N-doped reduced GO shows clearly n-type electron doping behavior with the Dirac point (DP) at negative gate voltages in three terminal devices. Our method could lead to the synthesis of bulk amounts of N-doped, reduced GO sheets useful for various practical applications.