R wave amplitude changes during exercise have been a controversial issue as both increase and decrease in amplitude have been reported in patients with coronary arterial disease. This variability in response is attributed to change in position and heart axis on exercise. In view of this limitation, this study evaluated the change in spatial R maximum amplitude on exercise, which should not be affected by the above factors. Twenty patients with ischaemic heart disease (male 20, age 38-61 years) and 9 control subjects (male 9, age 32-65 years) were studied. Orthogonal leads, X, Y, Z were recorded using corrected Frank lead system on a stereokinematic vectorcardiograph (Tonnies). The magnitude of spatial R maximum cardiac vector increased from 0.1 to 0.6 mV in 8/9 control subjects and decreased or showed no change in 18 of the 20 patients with coronary arterial disease. In the control group, the mean value at the end of exercise (0.98 +/- 0.34 mV) was significantly less (P less than 0.01) as compared to pre-exercise value (1.09 +/- 0.2 mV). Our preliminary observations thus indicate that, with exercise, the magnitude of spatial R maximum cardiac vector decreases or shows no alteration in height in patients with coronary arterial disease whereas it increases in normal subjects.