Field maps of the induced main magnetic field offset (DeltaB(0)) were measured in the rat heart at various points in the cardiac cycle for the purpose of identifying their effects on relaxation measurements. The mean DeltaB(0) of the left ventricle averaged across rats was found to be 0.11 +/- 0.35 ppm and 0.19 +/- 0.39 ppm at the onset of systole and diastole, respectively. The root mean square (RMS) variation in resonant frequency of the left ventricle averaged across rats was found to be 0.09 +/- 0.05 ppm and 0.06 +/- 0.04 ppm during systole and diastole, respectively. Temporal variations in DeltaB(0) could substantially affect quantitative MRI measurements. To assess this, transverse relaxation rates (R(2) and R(2)(*)) were measured at different points in the cardiac cycle, and the effects of DeltaB(0) were estimated using measured field map data. For a given region of the left ventricle, DeltaB(0) induced a mean error across rats of < or =3.9% for R(2) and < or =9.6% for R(2)(*). For R(2)(*) measurements, the static component of the field inhomogeneity was found to be responsible for most of the error induced. Copyright 2007 Wiley-Liss, Inc.