The rates of hydrogen exchange were measured in a "physiological" denatured state of staphylococcal nuclease using a NMR magnetization transfer experiment suitable for the measurement of exchange rates faster than 0.5 s-1. The results are compared with predicted exchange rates (kex) for the random coil state (Bai et al., Proteins 17:75-86, 1993). No protection factors (= predicted rate/measured rate) larger than 2.4 were observed, consistent with other NMR data which strongly suggest only small amounts of residual secondary structure in this denatured state. Systematically low protection factors (0.51 +/- 0.23) were found for Asp and Glu residues, while high protection factors were observed for Gly (1.60 +/- 0.60). We conclude that the predicted exchange rates (kex) may have an uncertainty of 2- to 3-fold. Thus, for denatured proteins only protection factors with a value of 5 or larger can be assigned structural significance. These results also demonstrate that multidimensional magnetization transfer NMR techniques are powerful tools in this research field due to its ability to measure rapidly exchanging protons (> 05 s-1) with high accuracy.