Carnitine plays an important role in fat metabolism. A long-echo time proton magnetic resonance spectroscopy protocol was implemented for detection of skeletal muscle acetylcarnitine during the day and after exercise on a clinical 7T scanner in the thigh (vastus lateralis) muscle. Our observation point towards diurnal changes of acetylcarnitine concentration which tended to be higher in the morning than after lunch. Moreover, following 10 minutes of high-intensity exercise the concentration significantly increased and again significantly decreased 15 minutes after cessation of the exercise. Our data emphasize the need for strict standardization,physical activity and dietary conditions for the measurement of the acetylcarnitine/carnitine.