Determination of 15N-NMR relaxation rates has allowed the characterisation of the dynamic properties of 55 1H-15N bond vectors in a soluble haem-binding domain of bovine microsomal ferricytochrome b5 consisting of the first 104 amino acid residues. Measurements of heteronuclear [1H]-15N-NMR nuclear Overhauser effects, and 15N-NMR longitudinal and transverse relaxation rates have been analysed using both model-free and reduced spectral density mapping approaches, demonstrating the application of these methods to a paramagnetic system. Analysis reveals that, for those regions which have been assessed, the polypeptide backbone of ferricytochrome b5 is largely rigid (average S2 = 0.80), and that minimal internal backbone motion occurs on the sub-nanosecond timescale. In contrast, motions on the microsecond to millisecond timescale, especially pronounced for the loop region extending from residues 28 to 31, and which may be of biological relevance, are indicated.