The influence of the N-15 CSA on N-15 longitudinal relaxation is investigated for an amide group in solid proteins in powder form under MAS. This contribution is determined to be typically 20-33%, of the overall longitudinal relaxation rate, at 11.74 and 16.45 T, respectively. The improved treatment is used to analyze the internal dynamics in the protein Crh, in the frame of a motional model of diffusion in a cone, using the explicit average sum approach. Significant variations with respect to the determined dynamics parameters are observed when properly accounting for the contribution of N-15 CSA fluctuations. In general, the fit of experimental data including CSA led to the determination of diffusion times (tau(w)) which are longer than when considering only an N-15-H-1 dipolar relaxation mechanism. CSA-Dipole cross-correlation is shown to play little or no role in protonated solids, in direct contrast to the liquid state case. (C) 2007 Elsevier Inc. All rights reserved.