The internal mobility of the protein eglin c is characterized with spectral density functions of the NH vectors obtained from heteronuclear NMR relaxation at multiple field strengths (7.04, 11.74, and 14.1 T). The spectral density functions, J(omega), describe the frequency spectrum of the rotational fluctuations of the XH bond vectors (15N-1H and 13C-1H). The spectral density-mapping approach [Peng, J. W., & Wagner, G. (1992a) J. Magn. Reson. 98, 308-332; Peng, J. W., & Wagner, G (1992b) Biochemistry 31, 8571-8586] permits the direct evaluation of J(omega) at the five frequencies 0, omega N, magnitude of omega H - magnitude of omega X, omega H, and magnitude of omega H + magnitude of omega X. The 15N-1H relaxation measurements from three field strengths on 15N-enriched eglin c resulted in 18 relaxation rate constants per NH bond and 13 unique evaluations of each NH spectral density function. Dynamic heterogeneity along the protein backbone is manifested most clearly in spectral density values at lower frequencies (< 100 MHz). The effective value of J(0), J(eff)(0), is the most sensitive probe of dynamics as it is affected by both rapid internal motions and slow chemical exchange processes. Low J(eff)(0) and J(omega N) values are correlated with fast amide proton-deuteron exchange rates; the converse, however, is not observed. Anomalies in J(omega H) and J(magnitude of omega H +/- magnitude of omega N) observed in the first applications of the spectral-mapping approach are now attributable to the high sensitivity of these values to small errors in the rate constants. These anomalies can be reduced by the use of a reduced spectral-mapping procedure. The use of multiple field strengths allows the identification of slow exchange processes manifested as an increase of J(eff)(0) with spectrometer field strength.