In 28 juvenile hypertensives (mean age 24 years) central haemodynamics was investigated at rest and during exercise. Hypertensives had significantly higher diastolic pulmonary artery pressure at rest than controls. Pulmonary diastolic pressure increased in parallel with cardiac output during exercise in hypertensives, in controls there was no relationship of pulmonary wedge pressure to cardiac output. Patients with diastolic pulmonary artery pressure higher than 12 mmHg at rest, or higher than controls during exercise, showed also signs of left ventricular hypertrophy on the ECG and/or X-ray. Those who were hyperkinetic and had systemic blood pressure greater than or equal to 150/90 at rest, had also significantly higher mean pulmonary artery pressure at rest. A correlation between total pulmonary, as well as pulmonary vascular and systemic resistances has been found at rest and during exercise in hypertensives; in controls, there was a correlation between the total pulmonary, as well as pulmonary vascular, and systemic resistances at rest; during exercise there was a correlation between total pulmonary, as well as postcapillary pulmonary and systemic resistances.