Plasmalemma thickness was determined in the shoot apex of the long day plant Spinacia oleracea during transition to flowering. This membrane was shown to become thicker a few hours following the extension of the light period. In plants submitted to 24 h of continuous light, the plasmalemma was 10 to 15% thicker than in cells from vegetative plants maintained in short days. After the extension of the light period the numerical density of particles exposed on the extracellular and protoplasmic fracture faces of the plasmalemma remains unchanged. The total sterol content of the apex was analysed in parallel and showed a net increase during transition to flowering. This change was observed at approximately the time of the plasmalemma thickening during the critical photoperiod. The results strengthen the suggestion that changes in plasmalemma might play an important role in the establishment of the floral state of the whole plan.