Adenylate kinase (AK) occurs as a family of isoforms in the short day plant Chenopodium rubrum and the long day plant Nicotiana tabacum cv. Havana 425. At least two AK isoforms are localized in the chloroplast, one in mitochondria and one in the cytosol. In both species, a light regime leading to floral induction provokes a significant decrease in the activity of one chloroplast AK isoform. This observation indicates changes in the chloroplast compartment upon floral induction. A functional interaction of chloroplast AK with chloroplast adenylate translocator is discussed.