Young bean plants were allowed to absorb a limited amount of labelled phosphorus and were then placed in a minus phosphorus solution. The labelled phosphorus content of one of the primary leaves was studied by a Geiger-Muller tube directly against the leaf. During the first few days of the experiment the labelled phosphorus content increased gradually up to a maximum value. It was followed by a decrease until a constant level was reached, provided the period was long enough. This decrease of the labelled phosphorus content could be stopped and even changed into a increase simply by placing the test leaf in the dark. These effects were obtained irrespective of the light conditions of the other parts of the plant. However, these conditions did affect the phosphorus status of the leaf. A further loss could be induced by darkening the other parts of the plant. Subsequent illumination of these parts caused the labelled phosphorus content in the experimental leaf to increase again. These results were thought to be consistent with the hypothesis of a continuous circulation of the phosphorus in the plant, involving a steady migration of labelled phosphorus into and out of the leaf, the latter being coupled to the stream of assimilates which is influenced by the light conditions of the leaf. The question of the role played by changes in specific activity was briefly entered into.