AbstractThis study is an extension of a recent work (Canet D, Barthe P, Mutzenhardt P, Roumestand C. 2001. A comprehensive analysis of multifield 15N relaxation parameters in proteins: Determination of 15N chemical shift anisotropies. J Am Chem Soc 123:4567–4576) dealing with the exploitation of the three classical 15N relaxation parameters (the longitudinal relaxation rate, R1, the transverse relaxation rate, R2, and the 1H‐15N cross‐relaxation rate, σNH) measured at several magnetic fields (at least two) in uniformly 15N‐labeled proteins (the chosen example is the C12A‐p8MTCP1 protein whose relaxation parameters have been measured at five different fields). R2 is iteratively corrected for exchange, so that the three parameters provide consistently the NH dipolar and 15N csa (chemical shift anisotropy) contributions. The dipolar spectral densities are analyzed, residue by residue, according to the functional form $$a_d + { b_d \over {1 + \omega^{2}\tau^{2}_{e\!f\!f}}}$$ (d for dipolar), where τeff is an (effective) correlation time associated with the slow motion(s) sensed by the NH vector at the level of the residue to which it belongs. In comparison with the Lipari‐Szabo approach, the coefficient bd can be identified with 2KdτeffS2, S being the generalized order parameter and Kd reflecting the amplitude of the dipolar mechanism (which, besides constants, involves $$1/r^{6}_{NH}$$); ad is associated with fast local motions. The csa spectral densities appearing in the expressions of R1 and R2 can be analyzed according to $${b_{csa\hphantom{j}}} \over {1 + \omega^{2}\tau^{2\hphantom{T}}_{e\!f\!\!f}}$$; bcsa; is proportional to bd (the proportionality factor depending on molecular quantities) and can yield 15N csa values from the rNH value and from the knowledge of the angle between the two relevant tensors. Uncertainties are such that it is impossible (at least with the present data) to conclude about a possible variation of 15N csa values along the protein backbone. © 2005 Wiley Periodicals, Inc. Concepts Magn Reson Part A 24A: 1–9, 2005.