Reflex and elastic properties of the triceps surae (TS) were measured on 12 male cosmonauts 28-40 days before a 3-6 month spaceflight [BDC], 2 or 3 days after Return [R+2/+3] and a few days later [R+5/+6]. H reflexes to electrical stimulations and T reflexes to tendon taps gave the reflex excitability at rest. Under voluntary contractions, reflex excitability was assessed by the stretch reflex, elicited by sinusoidal length perturbations. Stiffness measurements concerned the musculo-articular system in passive conditions and the musculo-tendinous complex in active conditions. Results indicated: 1) no changes (P>0.05) in H reflexes, whatever the day of test; 2) increase in T reflexes (P<0.05) by 57%, despite a decrease (P<0.05) in musculo-articular stiffness (11%) on day R+2/+3. T reflexes decreased (P<0.05) between day R+2/+3 and R+5/+6 (-21%); 3) increase in stretch reflexes (P<0.05) on day R+2/+3 by 31%, whereas it decreased (P<0.05) between day R+2/+3 and R+5/+6 (-29%). Musculo-tendinous stiffness was increased (P<0.05) whatever the day of test (25%). Links between changes in reflex and stiffness were also studied by considering individual data. At R+2/+3, correlated changes between T reflexes and musculo-articular stiffness suggested that, besides central adaptive phenomena, musculo-articular structures took part in the reflex adaptation. This mechanical contribution was confirmed when data collected at R+2/+3 and R+5/+6 were used, since correlations between changes in stretch reflexes and musculo-tendinous stiffness were improved. In conclusion, the present study shows that peripheral influences take part in reflex changes in gravitational unloaded muscles, but can only be revealed when central influences are reduced.