For conventional fast magic-angle spinning (MAS) frequencies (</=15 kHz), there is a known heating of the sample due to the friction between the rotor and the bearing gas. At the recently achievable ultrafast MAS frequencies of up to 35 kHz, a more dramatic heating effect is expected. Using the 119Sn NMR signal of the chemical shift thermometer Sm2Sn2O7, this frictional heating effect is quantified, and a calibration of the sample temperature under ultrafast MAS conditions is described. An empirical expression is given which allows the determination of the sample temperature as a function of the bearing gas temperature and the spinning frequency. Copyright 1999 Academic Press.