[en] The ¹⁵N steady-state magnetization in the presence of off-resonance rf irradiation is an analytical function of the T₁/T₂ ratio and of the angle between the ¹⁵N effective field axis and the static magnetic field direction. This relation holds whatever the relaxation mechanisms due to motions on the nanosecond time scale, and the size of the spin system. If motions on the micro- to millisecond time scale are present (fast exchange), the same observable depends also on their spectral density at the frequency of the effective field. The cross-peak intensity in each 2D ¹⁵N-¹H correlation map is directly related to the dynamic parameters, so that the characterization of fast exchange phenomena by this method is in principle less time-consuming than the separate measurement of self-relaxation rates. The theory of this approach is described. Its practical validity is experimentally evaluated on a ¹⁵N-labeled 61 amino acid neurotoxin. It turns out that existing equipments lead to non-negligible biases. Their consequences for the accuracy attainable, at present, by this method are investigated in detail