Freshly cut discs from growing potato tubers were incubated for 3 h with 10 mM orotate or 10 mM uridine. Control discs incubated without precursors showed a 30-40% decrease of uridine nucleotides, but not of adenine nucleotides. Orotate- and uridine-feeding led to a 1.5- to 2-fold increase in the levels of uridine nucleotides compared with control discs, and a 15-30% increase compared with the original values in intact tubers, but did not alter the levels of adenine nucleotides. Between 70-80% of the uridine nucleotides were present as UDPglucose, 15-25% as UTP, and 2-3% as UDP. The increase of uridine nucleotides involved a similar relative increase of UDPglucose, UTP and UDP. It was accompanied by a slight stimulation of the rate of [(14)C]sucrose uptake, a 2-fold stimulation of the rate at which the [(14)C]sucrose was subsequently metabolised, a small increase in the levels of hexose phosphates, glycerate-3-phospate and ADPglucose, and a 30% shift in the allocation of the metabolised label in favour of starch synthesis, resulting in a 2.4-fold stimulation of the rate of starch synthesis. Orotate led to a similar increase of uridine nucleotide levels in the presence of [(14)C]glucose, but did not significantly alter the rate of glucose uptake and metabolism to starch, nor did it increase the rate of sucrose resynthesis. The levels of uridine nucleotides were high in tubers on 6 to 10-week-old potato plants, and declined in tubers on 12 to 15-week-old plants. Comparison with the effect of the uridine nucleotide level in discs shows that the high levels of uridine nucleotides in tubers on young plants will play an important role in determining the rate at which sucrose can be converted to starch, and that the level of uridine nucleotides is probably co-limiting for sucrose-starch conversions in tubers on older plants.