Abstract. Time series measurements of six enzymes linked to different pathways in energy metabolism [NAD- and NADP-dependent glyceraldchyde-3-phosphate dehydrogenase (NAD- and NADP-GPDs), adenylate kinase (AK), malate dehydrogenase (MDH), glucose-6-phosphate dehydrogenase (G-6PDH) and 6-phosphogluconate dehydrogenase (6-PGDH)] have been performed in spinach leaves subjected to different photoperiodic treatments, inductive or not of the flowering process. It is shown that the vegetative and the floral states are characterized by specific enzyme patterns, respectively. Among the six enzymes measured, only three of them present a particular behaviour during the early period of flower induction and probably might be specific markers of this physiological change in the leaves. It is concluded that in a C3 plant such as spinach, floral induction in the leaves mostly involves a reorganization of the ‘vegetative' metabolic network possibly through a change in phase relationships in the different pathways in energy metabolism.