The effects of changes in posture on left ventricular (LV) diameter and function were studied by echocardiography in 14 healthy children. On changing from the supine to the standing position, enddiastolic LV diameter decreased by 13 +/- 5% (P < 0.001), heart rate increased (P < 0.05) and calculated stroke index (-37 +/- 11%, P < 0.001) and cardiac index (-32 +/- 14%, P < 0.001) fell. There was not a significant change in the echocardiographic measurement. % delta s, mean Vcf and max Vpwm (ns) but mean Vcf increased in relation to mean blood pressure in 3 patients, suggesting an increase in LV contractility. Squatting was accompanied by an increase in LV cavity dimension (P < 0.001), while heart rate fell slightly and calculated stroke index (%35 +/- 28%, P < 0.001) and cardiac index (+33 +/- 27%, P < 0.001) increased. Mean blood pressure increased by 19 +/- 18% (P < 0.01). There was again no significant change in % delta s, mean Vcf and max Vpwm. Most patients fell on the control (supine) blood pressure--mean Fcf curve; in 2 patients there was a residual increase in sympathetic tone and LV contractility.