Nuclear magnetic resonance and neutron activation analysis were used to measure manganese concentrations in leaves of Acer platanoides. Mn was found to accumulate in both the vacuoles and the chloroplasts, with more Mn (per unit area) in sun leaves than in shade leaves. No Mn was lost at senescence. Different seasonal patterns of Mn accumulation were found in sun and shade leaves. The quantity of chloroplast reserve Mn (bound to the outer surface of thylakoid membranes) increased rapidly in sun leaves from bud-break through midsummer, and then remained approximately constant through senescence. In shade leaves, however, the quantity of reserve Mn increased slowly, and at approximately a constant rate throughout the growing season.