We made continuous electrocardiographic recordings on magnetic tape during 15 episodes of ischemia in five patients with variant angina to determine the characteristics of the QRS changes. Orthogonal leads were used and the electrocardiograms were analyzed visually and by digital computer. Changes were quantified by subtracting baseline electrocardiograms from those obtained during ischemia. Large changes in the QRS occurred during ischemia but the waveform quickly returned to baseline when the episode subsided. In all patients there was prolongation of the QRS duration and an increase in QRS voltage during the terminal 40 msec of the waveform in the lead(s) showing the most marked ST displacement. The increase in the terminal QRS could be represented by a vector directed toward the ischemic zone. In a given patient the amplitude of ST displacement varied between episodes, presumably because of variation in the intensity of ischemia, but the QRS changes were directionally similar in each episode. In two patients there was also a smaller change involving the initial 40 msec of the QRS that could be represented by a vector directed away from the ischemic zone. To determine the possible mechanism for the electrocardiographic changes, ischemic episodes of 120 to 150 sec were produced in seven dogs and electrocardiographic recording and analysis techniques similar to those used in patients were employed. Myocardial conduction velocity was measured in three directions in the ischemic zone and was correlated with simultaneous electrocardiographic recordings from the body surface. The electrocardiographic changes in the dog preparation were virtually identical to those in the patients and strongly correlated with a fall in myocardial conduction velocity. We conclude that the QRS changes during variant angina result from the altered excitation pattern produced by conduction delay in the ischemic zone. The probable cause for the increase in terminal QRS voltage is delayed (and uncancelled) activation of the ischemic zone.