Affymetrix ATH1 arrays, large-scale real-time reverse transcription PCR of ∼ 2200 transcription factor genes and other gene families, and analyses of metabolites and enzyme activities were used to investigate the response of Arabidopsis to phosphate (Pi) deprivation and re-supply. Transcript data were analysed with MapMan software to identify coordinated, system-wide changes in metabolism and other cellular processes. Phosphorus (P) deprivation led to induction or repression of > 1000 genes involved in many processes. A subset, including the induction of genes involved in P uptake, the mobilization of organic Pi, the conversion of phosphorylated glycolytic intermediates to carbohydrates and organic acids, the replacement of P-containing phospholipids with galactolipids and the repression of genes involved in nucleotide/nucleic acid synthesis, was reversed within 3 h after Pi re-supply. Analyses of 22 enzyme activities revealed that changes in transcript levels often, but not always, led to changes in the activities of the encoded enzymes in P-deprived plants. Analyses of metabolites confirmed that P deprivation leads to a shift towards the accumulation of carbohydrates, organic acids and amino acids, and that Pi re-supply leads to use of the latter. P-deprived plants also showed large changes in the expression of many genes involved in, for example, secondary metabolism and photosynthesis. These changes were not reversed rapidly upon Pi re-supply and were probably secondary in origin. Differentially expressed and highly P-specific putative regulator genes were identified that presumably play central roles in coordinating the complex responses of plants to changes in P nutrition. The specific responses to Pi differ markedly from those found for nitrate, whereas the long-term responses during P and N deprivation share common and non-specific features.