Ab initio configuration interaction calculations were carried out on the potential-energy surfaces of the ground and Rydberg excited electronic states of NO2. The results show that potential-energy curves with typically Rydberg form are obtained for most of the excited states at linear geometries, similar to the ground-state potential of NO2+. At nonlinear geometries valence-Rydberg interactions complicate the potential-energy surfaces of the excited states. Quantum defect functions have been determined from the ab initio results on the 3d sigma Rydberg state and vibrational autoionization widths have been calculated for excited vibrational levels of members of the nd sigma series.