Beat-to-beat variations in the amplitude of the T wave (T wave alternans [TWA]) have been associated with malignant ventricular arrhythmias in patients with structural heart disease. TWA has also been observed sporadically in healthy persons during strenuous exercise. Therefore, it was the aim of this study to investigate the prevalence of TWA in healthy subjects at rest and during exercise. TWA was assessed in 48 healthy volunteers with a mean age of 30 +/- 8 years (21-53 years) using the CH2000 system for measurement of microvolt level TWA. TWA was not observed in any individual at rest. Short transient intervals of TWA were observed during exercise in five (10.4%) subjects. Sustained TWA was observed in two (4.2%) individuals. In one of these two individuals sustained TWA was recorded at heart rates > 110 beats/min. In the other TWA was observed at an onset heart rate of < 110 beats/min, and therefore, was considered alternans positive. The prevalence of exercise related sustained TWA in healthy, young individuals is low (2.1%). Short transient intervals of TWA were observed in about 10% of healthy volunteers. These transient episodes of TWA had no clinical impact with respect to a higher vulnerability to ventricular arrhythmias.