When suspended in media lacking free Mg(2+), chloroplasts from young pea plants (Pisum sativum CV Progress No. 9) lose 25 to 75% of their stromal Mg(2+) content to the medium, without breakage. This effect amounts for the inhibition of protein synthesis in the dark by ATP in excess of the Mg(2+) provided, since free ATP chelates Mg(2+). The rate of loss is from 1 to 4.5 microgram-atoms Mg(2+)/milligram Chl/hour; and depleted chloroplasts take up Mg(2+) from the medium at even faster rates, to a total amount not much more than that present originally (0.8 to 1.8 microgram-atoms/milligram Chl with an average of 1.33 +/- 0.32 mug-atoms/mg Chl). Leakage is completely prevented by 0.25 to 0.40 millimolar external Mg(2+). Addition of Mg(2+) at a level sufficient to prevent leakage from intact chloroplasts results in approximately 20% stimulation in light-driven protein synthesis.