ECG amplitudes and mean spatial vectors are analyzed according to levels of blood pressure in three blood pressure groupings among a sample of putatively healthy laborers who ranged in age from 20 to 60 years (Group A-under 140 90 mm. Hg blood pressure; Group B- 140 90 to 159 94 mm. blood pressure; and Group C- 160 95 to 199 109 mm. Hg blood pressure). In the groups with asymptomatic moderate elevation of blood pressure, significant differences from the "normotensives" are found, with lower T-wave amplitude, greater elevation of QRS and T vectors, and a widened spatial angle between QRS and T vectors. The differences are not due principally to the effect of overweight. The "normal age trend" toward a horizontal electrical heart position is not apparent in the "normotensive" group. The ECG and vectorial differences found are in the direction of those electrical features which characterize myocardial hypertrophy and ischemia, and they probably represent the earliest ECG signs of the myocardial effect of an increased cardiac work load.