The results of studies on the activity of lysosomal enzymes in different organs (liver, gills, muscles, gonads) of mature threespine stickleback Gasterosteus aculeatus L. at the beginning and in the end of the spawning period are reported. Fish were captured from three spawning grounds in the Gulf of Kandalaksha, White Sea, which differed in temperature, salinity, retention time, aquatic vegetation and other parameters. The samples were analyzed for the activity of five acid hydrolases (acid phosphatase, RNase, DNase, p-galac- tosidase, p-glucuronidase). During the spawning period, fish were shown to be in a state of high metabolic stress, as evidenced by an increase in acid phosphatase and glyco- sidase activity in the liver and especially a significant increase in the level of almost all the studied enzymes in the skeletal muscles of stickleback males and females from all habitats by the end of spawning. We are thus driven to the conclusion about a powerful effect of endogenous factors (sex steroid and othe