The effect on plasmalemma of exogeneously applied gibberellic acid (GA3) was investigated in spinach (Spinacia oleracea). Highly purified plasmalemma was obtained by free-flow electrophoresis and identified on the basis of both activity of the enzyme markers 1,3-β-glucan synthase and membrane morphology. Total lipids were extracted and the sterols separated by thin-layer chromatography (TLC). Thickening of the plasmalemma and modifications in its sterol content were observed in leaves in response to gibberellic acid treatment. The changes in sterol amount were quantified by reverse phase high performance liquid chromatography (RPHPLC). For these determinations pure plasmalemma was recovered by partionning in an aqueous two-phase system. The results were discussed with those previously reported for the plasma membrane of spinach induced to flowering by a transfer to continuous light.