Treatment of isolated chloroplasts with glutaraldehyde affects their ability to photoreduce artificial electron acceptors. The remaining rate of O2 evolution approaches zero with methyl viologen, is low with ferricyanide, but nearly normal with lipophilic Photosystem II acceptors, like oxidized p-phenylenediamine and oxidized diaminodurene. Since Photosystem I donor reactions are also affected, a specific site of inhibition of electron transport to Photosystem I is indicated. At the same time, glutaraldehyde prolongs the longevity of the chloroplasts stored in dark. In control samples the half-life of Photosystem II activity varied between 5 days at 4degreesC and 1 day at 25degreesC. Glutaraldehyde treatment increased these half times approx. 3-fold. The glutaraldehyde doses required to induce inhibition and stabilization were very similar.