Large-scale protein conformational motions on nanosecond-microsecond time scales are important for many biological processes, but remain largely unexplored because of methodological limitations. NMR relaxation methods can access these time scales if protein tumbling is prevented, but the isotropy required for high-resolution solution NMR is then lost. However, if the immobilized protein molecules are randomly oriented, the water H-2 and O-17 spins relax as in a solution of freely tumbling protein molecules, with the crucial difference that they now sample motions on all time scales up to similar to 100 mu s. In particular, the exchange rates of internal water molecules can be determined directly from the H-2 or O-17 magnetic relaxation dispersion (MRD) profile. This possibility opens up a new window for characterizing the motions of individual internal water molecules as well as the large-scale protein conformational fluctuations that govern the exchange rates of structural water molecules. We introduce and validate this new NMR method by presenting and analyzing an extensive set of H-2 and O-17 MRD data from cross-linked gels of two model proteins: bovine pancreatic trypsin inhibitor and ubiquitin. We determine residence times and order parameters of four internal water molecules in these proteins and show that they are quantitatively consistent with the information available from crystallography and solution MRD. We also show how slow motions of side-chains bearing labile hydrogens can be monitored by the same approach. Proteins of any size can be studied at physiological hydration levels with this method.