The effects of inorganic phosphate (Pi) deficiency on the expression of the UDP-glucose pyrophosphorylase (UGPase) gene (Ugp), involved in sucrose synthesis/metabolism, and on carbohydrate status were investigated in different tissues of Arabidopsis thaliana (L.) Heynh. For leaves, a decrease in internal Pi status caused by growth of plants on a medium lacking Pi (-P conditions) led to an increase in the overall content of glucose and starch, but had little effect on sucrose content. The Pi deficiency also led to an increased carbohydrate content in stems/flowers, but not in roots. The expression of Ugp was upregulated in both leaves and roots, but not in stems/flowers. The effects of Pi status on Ugp expression were confirmed using leaves of both pho1-2 and pho2-1 mutants of Arabidopsis (Pi-deficient and Pi-accumulating, respectively) and by feeding the leaves with D-mannose, which acts as a sink for Pi. The Pi-status-dependent changes in Ugp expression followed the same patterns as those of ApS, a gene encoding the small subunit of ADP-glucose pyrophosphorylase, a key enzyme of starch synthesis. The changes in Ugp mRNA levels, depending on internal Pi status, were generally correlated with changes in UGPase protein content and enzymatic activity. This was demonstrated both for wild-type plants grown under Pi-deficiency and for Pi mutants. The data suggest that, under Pi-deficiency, UGPase represents a transcriptionally regulated step in sucrose synthesis/metabolism, involved in homeostatic mechanisms readjusting the nutritional status of a plant under Pi-stress conditions.