The influence of salinity in the growing media on ribulose-1, 5-bisphosphate (RuBP) carboxylase and on CO2 fixation by intact sugar beet (Beta vulgaris) leaves was investigated. RuBP carboxylase activity was mostly stimulated in young leaves after exposure of plants for 1 week to 180 mM NaCl in the nutrient solution. This stimulation was more effective at the higher NaHCO2 concentrations in the reaction medium. Salinity also enhanced CO2 fixation in intact leaves mostly at rate-limiting light intensities. A 60 per cent stimulation in CO2 fixation rate was obtained by salinity under 450 μE m−2 s−1. At quantum flux densities of 150 μE m−2 s−1 (400–700 nm) this stimulation was 280 per cent. Under high light intensities no stimulation by salinity was found. In contrast, water stress achieved by direct leaf desiccation or by polyethylene glycol inhibited enzyme activity up to fourfold at −1.2 MPa.