Glutaraldehyde inhibits the flow of electrons in spinach chloroplasts, affecting the transfer between photosystems. The O(2) production and photosystem II are not impacted, but P700 reduction is severely hindered. Cytochrome f can still be reduced by photosystem II but oxidized slowly by photosystem I. Plastocyanin is sensitive to glutaraldehyde, suggesting that the inhibition occurs at the plastocyanin level, causing a disconnect between P700 and cytochrome f.