Changes in inorganic phosphate (Pi) concentrations in barley leaves during growth of plants with sufficient or deficient supplies of Pi were studied. Measurements of the Pi distribution from subcellular levels to the leaf tissue level under the same experimental conditions allowed us to analyse the relationship between the Pi homeostasis of various compartments and Pi re-translocation in the whole plant. Under Pi deficiency, the finding of growth-dependent changes in the Pi concentrations of whole leaves established that Pi was re-translocated from the older leaves to the young leaves. Translocation of 32Pi was also confirmed with an ‘imaging plate’ system, which made it possible to follow Pi movement in the same plantlet. To analyse the mechanism of Pi re-translocation, the Pi distribution amongst various compartments of the leaves was measured. Under Pi deficiency, the cytoplasmic Pi concentration of the first leaf remained constant until 16d after sowing, while vacuolar Pi was completely exhausted after 8 to 10d. Exhaustion of vacuolar Pi in the first leaf coincided with the appearance of the second leaf. The Pi concentration in the apoplast changed similarly to that of the whole leaf. However, the apoplastic Pi concentration was affected to some extent by the vacuolar Pi concentration and the growth of the younger leaf, because the main change in apoplastic Pi concentration coincided with the time of the disappearance of the vacuolar Pi and the appearance of the younger leaf. The Pi concentration in the apoplast was about 0.1 to I molm−3, even in the absence of Pi, which was much higher than that in the usual soil environment (a few mmolin−3). This suggests that the Pi absorbed by root cells is concentrated in the transport process from the root to the leaf apoplast. The content of Pi in the xylem exudate was constant irrespective of growth culture conditions. The root may be functioning as the constant Pi supplier to the above tissues.