The HMG-box sequence motif (approximately 80 residues) occurs in a number of abundant eukaryotic chromosomal proteins such as HMG1, which binds DNA without sequence specificity, but with "structure specificity", as well as in several sequence-specific transcription factors. HMG1 has two such boxes, A and B, which show approximately 30% sequence identity, and an acidic C-terminal tail. The boxes are responsible for the ability of the protein to bend DNA and bind to bent or distorted DNA. The structure of the HMG box has been determined by NMR spectroscopy for the B-domain of HMG1 [Weir et al. (1993) EMBO J. 12, 1311-1319; Read et al. (1993) Nucleic Acids Res. 21, 3427-3436) and for Drosophila HMG-D (Jones et al. (1994) Structure 2, 609-627]. It has an unusual twisted L-shape, suggesting that the protein might tumble anisotropically in solution. In this paper we report studies of the A-domain from HMG1 using 15N NMR spectroscopy which show that the backbone dynamics of the protein can be described by two different rotational correlation times of 9.0 +/- 0.5 and 10.8 +/- 0.5 ns. We show that the relaxation data can be analyzed by assuming that the protein is a rigid, axially symmetric ellipsoid undergoing anisotropic rotational diffusion; the global rotational diffusion constants, D parallel and D perpendicular, were estimated as 2.47 x 10(7) and 1.49 x 10(7) s-1, respectively. By estimating the angle between the amide bond vectors and the major axis of the rotational diffusion tensor from the family of structures determined by NMR spectroscopy [see accompanying paper, Hardman et al. (1995) Biochemistry 34, 16596-16607], we were able to show that the ellipsoid spectral density equation can reproduce the major features of the 15N T1 and T2 profiles of the three helices in the HMG1 A-domain. The backbone dynamics of the A-domain were then compared with those of the B-domain and the HMG box from HMG-D. This comparison strongly supported the differences observed in the orientation of helix I in the three structures, where the B-domain appears to be more similar to HMG-D than it is to the A-domain. These differences may turn out to be related to subtle differences in the DNA-binding properties of the A- and B-domains of HMG1.