The chemical changes taking place in enamel during carious attack have been studied by analysing the histologically defined zones of the small approximal lesion. Sam- pies of altered enamel were dissected from each zone and compared with sound enamel from adjacent control areas. Enamel from the primary translucent zone, the area of the lesion presumed to be least affected by caries, contained about 1.2% by volume less mineral and about 12% by weight less magnesium than the adjacent, apparently sound enamel. The translucent zone, therefore, appeared to result from demineralisation, associated with a preferential loss of magnesium-rich mineral.