Highly purified plasma membrane was obtained by phase partitioning and their relative content of sterol was investigated during flowering induction of spinach (Spinacia oleracea L. cv. Nobel). For this purpose, vegetative spinach was submitted to an inductive photoperiod or to a treatment with gibberellic acid (GA3). Quantitative and qualitative modifications of plasmalemma sterol content were observed during the first hours of an inductive photoperiod and after 24-h of continuous light. The main alteration of membrane sterols was in their saturation level. This change, obtained during critical photoperiod, was inhibited by 2-isopropyl-4-dimethylamino- 5-methylphenyl-1-piperidine carboxylate methyl chloride (AMO 1618). Modifications of plasmalemma sterols were also obtained after a 72-h gibberellic acid treatment of the plants maintained under short day. Some differences exist with those observed during inductive prolongation of light. These different results are discussed in relation with the previously reported thickening of plasmalemma induced by a transfer of spinach to continuous light and by a GA3 treatment.