1. Twelve subjects (mean age 46.3 +/- 12.5 years) with mild to moderate hypertension were studied before, during and after bicycle ergometer exercise. 2. Baroreflex sensitivity was determined by the Oxford phenylephrine method; sensitivity at rest was inversely related to intra-arterial pressure and age. Age and resting arterial pressure were not related. 3. Exercise for 5 min at 50 W and 5 min at 75 W raised the mean arterial pressure from 116.4 +/- 18.0 to 150.0 +/- 25.4 mmHg, the heart rate from 73.2 to 126.7 beats/min and the plasma noradrenaline from 541 +/- 142.7 to 1309.8 +/- 543.5 pg/ml (P less than 0.001). 4. The increase in noradrenaline during exercise and the maximum mean pressure achieved were inversely related to resting baroreflex sensitivity (r = -0.68 and -0.77 respectively). Resting values of noradrenaline were not related to baroreflex sensitivity, age, or resting blood pressure. 5. It is possible that the rise in both plasma noradrenaline and arterial blood pressure produced by exercise is controlled by the baroreceptor reflexes; these are less effective in hypertensive subjects and thus the increases in noradrenaline and arterial pressure during exercise are greater in subjects with raised blood pressure.