1. 1. Leaves of an albino mutant and of normal green seedlings of maize have high levels of readily extractable alkaline inorganic pyrophosphatase (EC 3.6.1.1) when grown under strong illumination. About one-tenth as much activity is found in both types of plants when grown in the absence of light. A brief illumination of dark-grown seedlings induces a large increase in readily extractable pyrophosphatase activity in the subsequent dark period. 2. 2. In light-grown maize the enzyme is localized in the chloroplasts but is readily washed out by aqueous solvents. The enzyme is highly specific for Mg 2+ and PP i, and has optimal activity between pH 8 and 9. Leaf extracts of sorghum have similar high activity; many other plants, and other maize tissues have much less activity.