Before tuberization, the radiocarbon from 14CO2 fixed by single leaves of Solanum tuberosum cv. Majestic was distributed fairly uniformly throughout the plant and treatment with gibberellic acid prior to 14CO2 fixation bad little effect on the distribution of labelled assimilates. After tuberization, the tubers imported the majority of the 14C supplied. Pretreatment with gibberellic acid greatly reduced accumulation of assimilate in, and growth of, tubers, particularly in the 7-day period following pretreatment. This effect was manifest in plants from which all aerial shoot-apices had been removed, suggesting that gibberellic acid acted directly on the tubers. The reduced import by tubers in gibberellic acid-treated plants was associated with a substantial decrease in the ratio, 14C in the ethanol-insoluble fraction to 14C in the ethanol-soluble fraction. It is proposed that the main effect of gibberellic acid is to decrease starch accumulation and increase sugar levels in the tubers, and that these changes are associated with the characteristic‘growing-out’ of the tubers and a ‘switching-off’ of the tuber sink.