The fate of ribulose-1,5-bisphosphate carboxylase was studied during development of carnation (Dianthus cryophyllus L. cv. White Sim) petals. The petals of various developmental stages were dissected into upper wide and basal narrow parts, and analyzad separately. Rubisco activity was assessed in aqueous extracts of the petal parts in parallel with measurements of CO2 fixation by intact petals. During development, CO, fixation declined markedly, Rubisco activity per corolla declined slightly and Rubi sco activity per g dry weight increased. Analysis of the steady state level of the large and small subunits (LSu, SSu) of Rubisco by western blotting, demonstrated a decline in the amounts of both. The reduction in the level of SSu appeared prior to the decrease in the amount of LSu. An analysis of the spatial distribution of the Rubisco subunits revealed that the upper part of the petal was devoid of SSu in both young and old flowers. The data suggest that two different mechanisms are involved in the regulation of the decline in the amount of LSu. The first is at the transcriptional level and operates primarily in the basal part of the petal. The second mechanism operates in the upper part of the petal and may involve post-transcriptional processes. 0 Elsevier, Paris Carbon fixation, / chlorophyll I ribtilose-1,5bisphosphate carboxylase 1 senescence I Dianthus caryophyllus