In goldfish Carassius auratus acclimated under critical salinity conditions (CS, 11.5 g kg -1 ), the concentrations of cortisol, glucose, Na + in the blood plasma increased significantly, also large β - HDL particles decomposed into small α- ones with a higher negative charge and a smaller number of apolipoproteins molecules in 1 particle composition. Particles remodeling led to ~1.5-2-fold increase in their number per unit plasma volume. Taking into account that under CS conditions the contribution of organic osmolytes to plasma osmolality increased by ~2%, and the content of the bound water fraction in plasma increased by ~4 times, we assumed that goldfish have a “reserve” mechanism for maintaining homeostasis: when the possibilities of increasing the concentration of Na + and organic metabolites in plasma are limited, remodeling of HDL particles and an increase in their amount in plasma can help to maintain its hyperosmotic status and additional influx of water into the body.