The temp. dependence of NMR relaxation rates was investigated for the backbone of 15N/13C labeled human ubiquitin in the temp. range of 20-50 DegC. The 15N autorelaxation rates give evidence that the potential energy functions for 15N-HN bonds are not quadratic, in agreement with results for other proteins. Cross-correlation rates arising from correlated fluctuations of two 15N-HN dipole-dipole interactions involving successive residues were obtained by the method of Pelupessy et al. (P. Pelupessy, S. Ravindranathan, G. Bodenhausen: J. Biomol. NMR 25, 265-280, 2003). The results suggest the presence of slow internal motions at 50 DegC.