In the blood plasma of 35 crew members, who completed expeditions to the International space station cystatin C concentra­tion was investigated in pre- and postflight periods. Applying Hawk's formula, the glomerular filtration rate (GFR) was calcu­lated. After short-term flights (from 9 to 13 days) the cosmonouts (group 1) revealed no changes in the level of cystatin C in the blood plasma and magnitude of GFR. In cosmonauts aged 35–48 years after the flights with duration from 134 to 215 days (group 2), on the 14^th day of the recovery period the level of cystatin C increased by 8%, and GFR decreased proportionally compared to the preflight values (p <0.05). In the members of the crew over the age of 50 years who participated in expeditions lasting from 134 to 198 days (group 3) a significant increase in the concentration of cystatin C in plasma and a reduced GFR was observed in all survey periods, beginning from the 1^st day of the recovery period, and in the dynamics of the re-adaptation those changes were more pronounced than in the cosmonauts under 50 years of age.