The effect of nitrogen in hydrogenated amorphous silicon (a-Si : H) has been investigated using thin films produced by r.f.-sputtering and by r.f. glow discharge decomposition of silane. In r.f.-sputtered a.-Si : H a donor action is observed and Ev may be moved to within 0·2eV of Ec. In contrast, a large nitrogen content in glow discharge a.-Si : H gives conductivity enhancement, but no doping effect. These results are discussed and compared with those from a-Si : H doped using other techniques.