Like the sieve tube sap of dicotyledons analysed so far, the phloem exudate of Yucca flaccida contains a high level of ATP, i.e. 615 μg/ml on the average. ATP concentration in the phloem exudate does not decrease during extended periods of exudation. 2. The pattern of labelled phosphorylated compounds after feeding of (32)PO 4 (---) is essentially similar in the phloem exudate of Yucca flaccida and in the exudate from the mouth parts of aphids feeding on Salix triandra phloem. 3. The label was incorporated very rapidly into most of the phosphorylated organic compounds present in the exudate of Yucca flaccida. in the first phase after feeding of (32)PO 4 (---) only the label in fructose-1,6-diphosphate and possibly the label in ATP was still increasing in relation to the total radioactivity. This increment was correlated to a loss of radioactivity in the inorganic phosphate fraction. 4. Specific radioactivity of inorganic phosphate and ATP in the sieve tube sap of Yucca flaccida increased continuously throughout the experiments while (32)PO 4 (-) was administered externally. This trend continued for some time after removal of the external source of label.
