Auroral NO chemistry is studied to determine the limits on [NO> and its production rate in steady auroral arcs. We conclude conservatively that the NO concentration cannot exceed 0.1[O2>. The precise value of this upper limit depends very sensitively on the relative yields of N(2D) and N(4S) in the reactions that are the major auroral sources of N atoms, and on the basis of the requirements of mid-latitude models for odd nitrogen, a valueof 0.03[O2> is estimated. Plausible auroral [NO> enhancements are of the order of (3--6) ¿108cm-3 after 103 s and (1--4) ¿109 cm-3 after 104 for a steady IBC 2+ arc.