A set of weak complexes between imidazole (Imi) and nitric oxide (NO) were calculated with UMP2/6-31++G(d,p) and UMP2/6-311++G(2d,2p) levels of theory. Planar and nonplanar geometries were considered. Complexes of NO and protonated imidazole (ImiH(+)) were also studied due to the biological important effect of Imi protonation. It was found that ring protonation increases the stability of planar complexes and does not affect significantly nonplanar complexes. The Z-H center dot center dot center dot XY (Z = C, N and X, Y = N, O) interactions resulted as hydrogen bonds according to Koch and Popelier criteria based on AIM theory. Charge transfer was also found very important for complex stabilization within Our theoretical framework. Planar NO center dot center dot center dot ImiH(+) complexes are more stable than those obtained with neutral Imi.