Leg venous measurements were performed in two cosmonauts during the 379-d "Mir" mission (K-1 and 340-d ISS mission (K-2). The paper presents individual changes in venous capacity, distention and filling rate in the legs of these two cosmonauts, and 23 cosmonauts on 6-mos. missions as controls. In the first half of the year-long missions, changes in venous characteristics showed similar trends though had different dimensions. For instance, in K-1 increases in capacity and distention were more pronounced than in K-2. In the second half of the missions, the cosmonauts again displayed different changes in venous characteristics. In K-1 they all were unstable with episodes of simultaneous increases in venous capacity, distention and filling rate that were unfavorable for orthostatic stability (OS). Changes in the K-2 leg veins were less dramatic and detrimental to OS. Recovery of the pre-flight status of leg veins following the year-long missions took a longer time as compared with 6-mos. missions; the process was still incomplete by day 8 after landing. We surmise that in future one-year and longer missions the majority of cosmonauts are likely to have favorable changes in leg veins; however, untoward venous developments and consequent OS impairment must not be excluded.