The uppermost layer of the epidermis, the stratum corneum (SC), was spin-labeled with a sulfhydryl-specific nitroxide reagent to investigate the water content effects upon the protein dynamics directly in the intact tissue. A two-state model for the nitroxide side chain described the coexistence of two spectral components in the electron paramagnetic resonance (EPR) spectra. The so-called strongly immobilized component, S, is associated with the EPR signal of a motionally restricted nitroxide fraction having its N-O group hydrogen bonded to protein (rigid structure) while the weakly immobilized component, W, corresponds to the signal provided by the spin labels with higher mobility (approximately 10 times greater) exposed to the aqueous environment. The relative populations between these two mobility states, S and W, are in thermodynamic equilibrium. The standard Gibbs free energy, enthalpy and entropy changes for transferring the nitroxide side chain from the state contacting the solvent, W, to the one contacting protein, S, indicated that the reduction of the SC water content to below approximately h 0.69 g H(2)O/g dry SC, stabilizes the protein interacting state, S. Upon decreasing the SC hydration level below approximately h 0.69 the segmental motion of the polypeptide chains and the rotational motion of the spin-labeled side chain were also constrained. This work can also be useful to improve the spectral analysis of site-directed spin labeling, especially for a more quantitative description in terms of thermodynamic parameters.