Myocardial T1 and T2 mapping allow for non-invasive quantification of myocardial tissue alterations. Several confounders can influence correct relaxation time assessment. However, the effect of physiological changes in myocardial water content has not been investigated yet. In an intraindividual cardiac magnetic resonance imaging (MRI) study, we investigated 12 healthy volunteers at baseline, after 10-12 hours of fasting and after rehydration. Especially after dehydration, T1 and T2 relaxation times were reduced compared to baseline indicating a detectable effect of the hydration status on relaxation time assessment. Hydration status should therefore be recognized as a possible confounder of T1 and T2 mapping.