The most important maintenance processes in plants are protein turnover and active transport processes to maintain certain ion concentrations in the cells. In this paper an attempt is made to calculate the total energy cost of these processes from what is known about their specific costs and what has been observed about their rates. Because of insufficient reliable data about rates of individual maintenance processes, only approximate values can be obtained. The average turnover rate of leaf proteins may be about 100 mg protein per g proteins per day at normal temperature in leaves assimilating at moderate light intensities. This process consumes 28–53 mg glucose per g protein per day, which equals 7–13 mg glucose per g dry weight per day in leaves. It is likely that the rates of protein turnover and of CO2-assimilation are related. The cost of maintaining ion concentrations is estimated to be about 6–10 mg glucose per g dry weight per day in leaves. The sum of these figures is lower than is indicated by measurements of maintenance respiration. One reason for the underestimation may be that the protein turnover rates used in the calculations apply to plants with lower photosynthetic rates than the plants in which the maintenance respiration was measured. Effects of water stress and salinity, temperature and other environmental factors on the rate of maintenance processes are discussed. The consumption of assimilates for maintenance of plant cells is a significant, negative factor in plant productivity. A better understanding of the maintenance processes may give a clue how to manipulate plant characteristics or the environment to reduce the amount of assimilates consumed in these processes. It is suggested that reduction in protein turnover rates may be one such manipulation.