Tha cause of ST-T changes with QRS high voltage in essential hypertension was investigated using Frank lead vectorcardiograms. Submaximal treadmill exercise was performed according to the modified Bruce protocol. The patients were divided into three groups. Group N consisted of 29 normal subjects; Group I (ischemic heart disease) consisted of 10 patients with angina pectoris and 16 patients with angiographically documented narrowing of one or more coronary arteries and Group H (essential hypertension) was subdivided into four groups according to ECG findings: Group H-1 of 30 subjects with normal ECG, Group H-2 of 20 subjects with QRS high voltage (Sv(1)+Rv(6)≧35mm) with normal ST-T pattern, Group H-3 of 20 subjects with QRS high voltage with ST-T changes but no "strain pattern" and Group 4 of 20 subjects with QRS high voltage and "strain pattern". After exercise, the maximum spatial QRS vectors shifted posteriorly and to the left in all groups. In Group N, the maximum spatial T vector was reduced in magnitude, and directed anteriorly, superiorly and to the left after exercise. The T loop configuration showed no significant changes, though slightly rounded and inscribed uniformly. In Group I, the maximum spatial T vector decreased in magnitude as in Group N, but was directed in significantly more ways than in Group N. In a few cases, the vectors were extremely dislocated upward. The T loop configurations had wide variation. This variation of the T loop after exercise was characteristic of Group I. In group H, the maximum spatial T vector was also reduced in size, while the azimuth, elevation and spatial QRS-T angle moved in the opposite direction of Group N, as cardiac hypertrophy beveloped. Analysis of T loop configuration changes induced by exercise strongly suggested that the ST-T changes in Group H-3 were related to primary changes due to myocardial ishemia. Effects of exercise on spatial T vectors and T loop configurations in Group H-4 were quite different from those in Group I, so that the ST-T changes in this group seemed to be secondary changes in association with QRS high voltage.