The time course of changes in VFT was determined during the 1st phase of arrhythmia following coronary occlusion and during consecutive reperfusion in five repeated periods of occlusion and reperfusion in 10 mongrel dogs (17--24 kg). VFT was determined using a square wave pulse series of 140 ms duration which was triggered by the R-wave of the ECG and placed into the vulnerable period of the cardiac cycle. After acute occlusion VFT decreased to a minimum level within a few minutes and then increased again slowly up to the control value which was reached about 20 min after the ligation. When the occlusions were repeated several times the extent of the decrease in VFT became increasingly less and its duration increasingly shorter until finally there was no significant decrease in VFT. Reperfusion after coronary occlusion led to an abrupt decrease in VFT within 1 min, followed by a rapid increase to the control value. This time course did not depend upon the number of prior occlusions. The results show that in the case of repeating short-term coronary occlusions one cannot expect comparable VFT time courses for the consecutive periods of occlusion except for the 1st and 2nd ones. Differing mechanisms leading to the occurrence of VF after coronary ligation and during reperfusion are discussed.