The effects of aluminum ions on the generation of mobile inorganic phosphate (Pi) within the cells of excised maize (Zea rnays L.) root tips were examined using 31p-nuclear magnetic resonance (31p-NMR) spectroscopy. When perfused with a solution containing 50 mM glucose and 0.1-5.0 mM Ca 2 + at pH 4.0, 3-5-ram-long excised maize root tips from 3-d-old seedlings showed a significant (approx. 100%) increase in the amount of mobile Pi, (primarily vacuolar) over a period of 30 h. This increase was above that which can be accounted for by the hydrolysis of endogenous sugar phosphates and nucleotides. A change of the pH of the perfusion solution to 7.0 reduced the increase in Pi to approx. 50%. Omission of Ca 2 + in the solution at pH 4.0 caused the mobile Pi to increase to about 170%. However, the presence of A13+ or both Ca 2 Β§ and AI 3+ in the solution resulted in a significant loss (35-50%) of mostly vacuolar Pi over the same period of time. When root tips containing up to 65% of newly released Pi, produced after 20 h perfusion, were exposed to A13 +, no additional increase in the level of the mobile-Pi signal area was noted. Exposure to A13 Β§ with Ca 2 Β§ and glucose under hypoxia at pH 4.0 resulted in a threefold decrease in intracellular Pi content after the root tips were returned to aerobic conditions. These results indicate that external pH plays an important role in the generation of mobile intracellular Pi and that the presence of both Ca 2 + and A13 Β§ can independently suppress the production of this excess Pi and ultimately reduce the vacuolar Pi.