Diurnal variations in the pattern of movement of sugars, water, and ions into developing tubers of the potato (Solanum tuberosum L.) were investigated. It was demonstrated using a recording balance that large increases in the fresh weight of tubers occur during a dark period of reduced transpiration. Movement of assimilated 14C did not reflect similar large changes and much of the weight change observed is considered to be fluctuations in tuber water content. This water was shown to be moving predominantly through the xylem of the stolon by introducing labelled ions, 32P and 89Sr into the plants. 32P, which moves in both xylem and phloem, was transported to the tuber at a constant rate whereas 89Sr, which behaves like calcium and is relatively immobile in the phloem, only moved into the tuber during the dark period. As well as the over-all long-term diurnal fluctuations several smaller rapid changes were recorded in the rate of water movement. Switching from darkness to light caused a transient increase followed by a rapid decrease in tuber weight. Switching from light to darkness caused a rapid increase in tuber weight. In some experiments small oscillations in tuber weight were recorded. The possibility of these oscillations being directly related to cyclic changes in transpiration is considered. The results are discussed in relation to solute movement within plants.