Sets of discs were taken from leaves of destarched tobacco plants (Nicotiana tabacum L. cv. xanthii) and floated on solutions of sucrose or glucose in the dark. Abundant starch was formed in the youngest leaves but there was a marked decline with leafage. By contrast, when replicate sets of discs were floated on water and illuminated, photosynthetic starch formation was similar in the differently aged leaves. Uptake of sugar, measured by dry weight increases and incorporation of [14C]sucrose, was not dependent on leaf age. The possibility that physiological changes, relating to ageing and import/export status of the leaf, regulate the metabolism of sugar to starch was examined. Increasing retention of sugar in the minor veins is likely to be a major factor. Invertase activities were measured and found to be similar in the differently aged leaves. Respiration rates declined with increasing leaf age. Speculations concerning changes in selective permeability of the chloroplast membrane are also discussed.