Thirty-two pairs of monozygotic and 21 pairs of dizygotic male twins aged between 18 and 31 years were studied. Blood pressure was measured and Doppler echocardiography at the level of the aorta was performed in resting conditions and at two levels of supine submaximal bicycle exercise (at a fixed work load of 60 W and at a work load corresponding to a heart rate of 110 beats/min). In resting conditions, a genetic component in the variability of systolic and diastolic blood pressure was found; during submaximal supine exercise only a minor genetic effect was observed. At rest, genetic variance was shown for left ventricular outflow haemodynamics and for peripheral vascular resistance. However, during exercise, the transmissible effect on haemodynamics and peripheral resistance was small.