Some marine immigrants into the Baltic Sea were studied with respect to the question whether salinity changes may lead to long-term stress effects. The O:N ratio was used as indicator of stress. It was calculated from measurements of oxygen consumption and excretion of ammonia and total nitrogen after long-term changes of salinity. In Mya arenaria, increased nitrogen excretion (ammonia and total nitrogen) at low salinity leads to a clearly reduced 0:N ratio, indicating stronger participation of proteins/amino acids in catabolism. This could contribute to reduction of growth rates and final size of the organisms under these conditions. In Asterias rubens of the western Baltic Sea, no correlation of size reduction and increased metabolism of proteins could be found. Starfish from the North Sea and the Baltic do not demonstrate significant differences of their 0:N ratios at the different salinities of their habitats. In Arenicola marina, which demonstrates only slight size reduction in the Western Baltic Sea, protein degradation does not seem to be affected by salinity. However, the animals excrete more total nitrogen but less ammonia at higher salinities. In this species the mode of excretion seems to change after salinity adaptation.