The activity of NAD+ and NADP+-dependent glyceraldehyde-3-phosphate dehydrogenase isozymes (EC 1.2.1.12 and EC 1.2.1.13, respectively) were measured in spinach (Spinacia oleracea L. cv. Nobel) leaves grown under different photoperiodic treatments in order to discriminate between the early events of floral induction and processes related to acclimation. Glycolysis-linked isozyme activities were increased not only during floral induction and acclimation, but also during acclimation alone, suggesting that the changes in cytosolic activities were most probably associated with acclimation. In contrast, the chloroplast-linked isozyme activities only increased during flower induction and appeared to be specifically associated with the initiation of the flowering process. The relative activity changes in the chloroplast and cytosol compartments may thus be supposed to be among the first signs of translation of the photoperiodic signal into cytosolic and cellular metabolic adaptation, whereby the leaf moves rapidly into a new metabolic state.