Ribulose-l,5-diphosphate (RuDP) carboxylase protein and activity were determined in relation to net photosynthetic rate during the senescence of intact flag leaves of wheat on the plant. Initially the decrease in RuDP carboxylase activity was greater than the decline in net photosynthesis. The major decrease in RuDP carboxylase activity over this period resulted from a decrease in enzyme specific activity from 11 to 2 μmol CO2 fixed h−1 mg−1 protein. Loss of RuDP carboxylase protein did not occur until late in senescence by which time chlorophyll concentration had decreased by more than 50%. Treatment of flag leaves at weekly intervals with either 1000 parts 10−6 2-chloro-ethyltrimethylammonium chloride or 100 parts 10−6 gibberellic acid with 1 part 10−6 kinetin did not significantly affect net photosynthetic rate, RuDP carboxylase protein or activity during senescence.