Direct quantitative measurements of both basic ventricular threshold and ventricular fibrillary threshold were made in dogs prior to and during progressive immersion hypothermia. No change in basic ventricular threshold was observed as rectal temperature fell from 38°C to 22°C. At the same time, ventricular fibrillary threshold showed a 5-fold decrease. Neither maintenance of a normal serum pH during cooling nor prolonged cold exposure prior to the hypothermic episode altered the decrease in ventricular fibrillary threshold.