Water oxygen-17 magnetic relaxation is shown to be a powerful technique for studying protein hydration. Longitudinal and transverse 170 relaxation rates were measured at variable frequency (4-35 MHz), temperature, pH, and protein concentration in aqueous solutions of seven proteins. The data were analyzed in terms of a fast exchange two-state model with local anisotropy. A water 170 quadrupole coupling constant of 6.67 MHz and an order parameter of 0.06 (from I7O splittings in lyotropic liquid crystals) results in approximately two layers of hydration water having a reorientational rate less than 1 order of magnitude slower than that of bulk water. This rapid local motion has a small anisotropic component, which is averaged out by protein reorientation with a correlation time of the order of 10 ns. Due to electrostatic protein-protein interaction the protein reorientation is considerably slower than predicted by the Debye-Stokes-Einstein equation. Charged residues, particularly carboxylate, are more extensively hydrated than other residues, accounting for the variation in the extent of hydration between different proteins.