A previous two-dimensional (2D) ultrasound study suggested that there is relaxation of the myocardium after defibrillation. The 2D study could not measure activity occurring within the first 33 ms following the shock, a period which may be critical for discriminating between shock-induced, and excitation-induced, relaxation. The objective of our study is to determine the left ventricular (LV) geometry during the first 33 ms following defibrillation. Biphasic defibrillation shocks were delivered 5-50 seconds after the induction of ventricular fibrillation in each of seven dogs. One-dimensional, short-axis ultrasound images of the LV cavity were acquired at a rate of 250 samples/second. The LV cavity diameter was computed from 32 ms before to 32 ms after the shock. Pre-shock and post-shock percent changes in LV diameter were analyzed as a function of time using regression analysis. The normalized mean pre- and post-shock slopes (0.2±2.2 and 3.3±7.9 % per 10 ms) were significantly different (p<0.01). The post-shock slope was positive (p<0.005). Our results confirm that the bulk of the myocardium is relaxing immediately after defibrillation.