Quantitative magnetic resonance imaging (MRI) was applied to assess structural and functional parameters of the rat heart in vivo. Using ECG and respiratory triggering, MR images were obtained at different time points during the cardiac cycle. This allowed accurate determinations of the left ventricular (LV) mass, wall thickness, LV end-systolic and end-diastolic volumes, stroke volume, and ejection fraction. LV mass determined by MRI showed and excellent linear correlation with post mortem gravimetric determination of LV weight. MRI was then used to examine the pathophysiological changes in two models of LV hypertrophy. In one group of animals the aortic arch was banded to an outer diameter of 1.0 mm to elicit a pressure overload on the LV. A second group was subjected to a volume overload due to graded disruption of the aortic valve. Although both models exhibited a similar degree of LV hypertrophy as shown by the LV weight/body weight ratio, important functional and structural differences were revealed by MRI. Aortic stenosis resulted in an increase in wall thickness, whereas stroke volume and ejection fraction did not differ compared to control animals. In contrast, aortic valve insufficiency did not affect LV wall thickness, however, LV chamber volume as well as stroke volume were markedly increased. Ejection fraction was significantly reduced in these animals. In conclusion, MRI allows the reliable in vivo determination of important structural and functional parameters of hearts in small rodents.