The effect of a six-week strenuous exercise training programme (modified Bruce protocol, treadmill, three times per week) on resting and exercising blood pressure, heart rate, plasma catecholamines, chromogranin A, renin activity and aldosterone levels was investigated in 15 patients with mild hypertension. An identical exercise test was conducted at baseline and study close (six weeks). At follow-up, seven to ten days after study close, patients completed an exercise test of equivalent intensity to that at baseline, achieving comparable heart rate levels at maximal exercise. On each occasion, blood pressure, heart rate and hormonal variables were measured at rest (supine), maximal exercise and ten minutes after stopping exercise. Resting and exercising blood pressure and heart rate were reduced by the six-week exercise regimen. There was a trend, although not statistically significant, for resting plasma noradrenaline levels to be lower at study close. The reduction in blood pressure and heart rate at maximal exercise was associated with a significant attenuation of the plasma renin response to exercise. Plasma catecholamines also appeared to be lower after exercise training, although this effect was not statistically significant. Plasma levels of chromogranin A and aldosterone measured at rest and maximal exercise were not influenced by the exercise regimen. Further controlled studies are required to corroborate the results of this preliminary study.