Neuroglobin (Ngb) is a globin expressed in the nervous system of humans and other organisms that is involved in the protection of the brain from ischemic damage. Despite considerable interest, however, the in vivo function of Ngb is still a conundrum. In this paper we report a number of kinetic experiments with O2 and NO that we have interpreted on the basis of the 3D structure of Ngb, now available for human and murine metNgb and murine NgbCO. The reaction of reduced deoxyNgb with O2 and NO is slow (t(1/2) approximately 2 s) and ligand concentration-independent, because exogenous ligand binding can only occur upon dissociation of the distal His-64, which is coordinated to the ferrous heme iron. By contrast, NgbO2 reacts very rapidly with NO, yielding metNgb and NO3- by means of a heme-bound peroxynitrite intermediate. Steady-state amperometric experiments show that Ngb is devoid of O2 reductase and NO reductase activities. To achieve this result, we have set up a protocol for efficient reduction of metNgb using a mixture of FMN and NADH under bright illumination. The results are discussed with reference to a global scheme inspired by the 3D structures of metNgb and NgbCO. Based on the ligand-linked conformational changes discovered by crystallography, the pathways of the reactions with O2 and NO provide a framework that may account for the involvement of Ngb in controlling the activation of a protective signaling mechanism.