We studied age and size dependence of pumping and respiration rates, mortality during prolonged air exposure and the changes in behavioural response to cyclic salinity fluctuations combined with elevated temperatures in the White Sea blue mussels Mytilus edulis L. Five age groups (2-9 years) and three size groups: small (S, mean wet tissue mass 0.12 g), medium (M, mean wet tissue mass 0.39 g) and large (L, mean wet tissue mass 0.78 g) were analysed. Respiration rate was measured in closed respirometers. Pumping rate was determined by a laser beam to monitor the changes in water level when the inhalant and exhalant siphons were in separate compartments of the aquarium. To study their response to environmental stress, the mussels were subjected to cyclic salinity fluctuations at normal (10°C) and elevated (20°C) temperatures. Two parameters were noted: (1) the salinity at which shell valves either opened or closed at both temperatures, and (2) intra-individual variation of the behavioural response to salinity changes at both temperatures. Size and age affect physiological processes in the blue mussel. The responses of M. edulis to environmental stress including low salinity, elevated temperature and air exposure as well as respiration rate are generally size-dependent. Effects of age on sensitivity to low salinity and survival in air are minor. Pumping rates of mussels are significantly affected by both size and age of the animals. The disproportional decrease of pumping rate is observed at the age of about 7-8 years old.