Exchange rates of rapidly exchanging (>1.0 s(-1)) backbone amide protons with solvent water in staphylococcal nuclease (SN) were measured at pH 6.03-7.03 with a 2D) heteronuclear water exchange filter sequence (WEX II-FHSQC). Comparison of the exchange data with crystal structure data reveals the following : (1) Nonhydrogen-bonding and exposed residues have protection factors (predicted exchange rate in random coil-measured exchange rate) of less than 15 for non-hydrophobic residues and 10 or higher for hydrophobic residues. Among non-hydrophobic residues, Gly tends to have a higher protection factor (10-15), whereas other residues are below 10. (2) Protection factors for buried and non-hydrogen-bonded protons vary over a wide range (6-10(4)). Low protection factors (<25) may indicate fluctuations in structure resulting in substantial solvent exposure. (3) Some hydrogen-bonded and buried protons show rapid exchange, and the protection factors are 25-400. This indicates kinetically labile hydrogen bonds and solvent exposure by structural fluctuations. (an the other hand, many protons in this category exchange slower than the detection limit (<1.0 s(-1)) and are mainly observed in alpha helices and beta sheets, or hydrophobic residues, Although a good correlation between NMR exchange measurements and X-ray structural properties was observed, discrepancies were also found for several residues, namely (80)Gln, (82)Thr, and (138)Asn. The measured pH dependence revealed unusual behavior for (77)Asn, possibly due to a tightly bound water molecule. Our data indicate that it is possible to use the exchange rates of rapidly exchanging protons as a probe for studying hydrogen bonding, solvent accessibility, or regional kinetic lability of protein structures in solution at physiological pH.