This work explores the physical basis for Hart's mechanical equation of state in high-temperature plasticity. The experiments seek to identify a possible microstructural basis for the hardness parameters associated with load relaxation curves. The experiments also seek to examine the microstructural basis for scaling in load relaxation data and to explore the relationship between creep and load relaxation. Constant stress creep and load relaxation tests were conducted on <100> oriented single crystals of halite at 700¿C and stresses between 0.6 and 3 MPa. Load relaxation tests were performed at 400¿C up to a stress level of 13 MPa. After testing, specimens were sectioned, and dislocation densities and subgrain size distributions were measured. Results at 700¿C reveal that distributions of subgrain size in crystals crept at different stress levels are similar to each other; that is, they have the same shape but different average subgrain sizes depending on stress level. Hardness curves obtained from load relaxation experiments at different levels of work hardening were found to correspond to different average subgrain size. Load relaxation data from 700¿C and 400¿C belong to a single-parameter family of curves, with hardness curves translating onto each other with a scaling slope m = 0.33 ¿ 0.05. Subgrain size distributions generated in creep are statistically identical to those from load relaxation. The hardness parameter, σ*, specified as the (apparent) high-strain rate limit of stress in the load relaxation data, is approximately 50Gb/DI, where G is the shear modulus, b is the Burgers vector, and DI is the mean intercept subgrain diameter. During creep under constant stress the subgrain size evolves until a steady value is approached. The experimental data lend credence to Hart's interpretation that load relaxation data represent (nearly) constant structure with subgrain size playing the role of the structural variable.