To secure cerebral perfusion, the giraffe heart generates a blood pressure that is twice as high as other mammals. Myocardial wall stress is normalized by a pronounced thickening of the left ventricular wall of the giraffe as compared to other species. However, since the relative heart mass of giraffes is similar to that of other mammals (0.6% of body mass), we hypothesized that the giraffe heart exhibits smaller left ventricular end-diastolic volumes. In a-chloralose anaesthetized giraffes we determined a cardiac output and a stroke volume that are considerably lower than similar-sized mammals (500 kg). By means of intravascular echocardiography, we measured typical mammalian ejection fractions and normal myocardial function, but low end-diastolic and end-systolic volumes. The ability of the giraffe heart to generate high pressures, therefore, seems to reside in a decreased ventricular radius and a thickened left ventricular wall that normalizes wall stress. As such, the myocardial adaptation to the high afterload with increased blood pressure entails a reduction in stroke volume that may render giraffes unable of sustaining a high cardiac output.