Focused ultrasound (US) surgery has been used to induce high temperature elevations in tissue to coagulate the proteins and kill the tissue. The introduction of noninvasive online temperature monitoring has made it possible to induce well-controlled thermal exposures. In this study, we used magnetic resonance imaging (MRI) thermometry to monitor thermal exposures near the threshold of tissue damage, and then investigated if apoptosis was induced. Rabbit brains were sonicated with an eight-sector phased array to create a large region of uniform temperature elevation at the end of a 30-s sonication. Histological examination demonstrated that apoptosis was induced in some cells. At 4 h after the sonications, the apoptotic cells constituted 9 +/- 7% of identifiable cells. By 48 h after the sonications, the number of apoptotic cells had increased up to 17 +/- 9%. The impact of this finding for therapy needs to be explored further.