M-mode echocardiograms were performed in 31 normal children in order to compare myocardial wall dynamics in different regions of the left ventricle. Tracings were recorded at the level of the mitral valve ring (level 1), near the tips of the mitral valve leaflets (level 2), and in the region of the papillary muscles (level 3) and were digitised. Fractional shortening increased from 31.7 per cent at level 1 to 36.5 per cent at level 3. Peak VCF and minor dimension lengthening rate were the same at all three levels, as were end-diastolic posterior wall thickness and peak systolic wall thickening rate. Striking regional differences, however, were seen in the extent of systolic posterior wall thickening which increased from 55 per cent at the base to 106 per cent at the papillary muscles, and in peak posterior wall thinning rate, which increased from 6.4 cm/s at the base to 10.0 cm/s at the papillary muscles. There was greater systolic inward movement of epicardium at level 1 than at levels 2 and 3. The septum thinned and thickened more slowly than did the posterior wall and did not show major differences between levels. Systolic reduction in minor dimension thus occurs largely by thickening of the wall at level 3 and by sphincter-like inward movement of the entire wall at level 1. These regional differences can be explained by a circumferential arrangement of myocardial fibres at the base of the heart and a more prominent longitudinal component towards the apex. Thus, non-uniformity of function is a prominent feature of the normal left ventricle and may reflect regional variation in its structure.