Summary In studies on the distribution of lead in rats given drinking water containing lead acetate at concentrations: 0, 100, 500, 1000 and 5000 μgPb/l for one year, no change was observed in the organ distribution of lead, or in the subcellular distribution of lead in the organelles in the heart. The mitochondrial enzyme, ferrochelatase, was significantly depressed in both the 500 and 1000 μg/l groups as was the cytoplasmic enzyme δ-aminolaevulinic acid dehydratase. These depressions of enzyme activity were associated with ultrastructural changes in the myocardium after six months, and were most pronounced in the mitochondria.