Abstract The function of the sarcoplasmic reticulum (SR) was examined in the slow soleus and fast extensor digitorum longus (EDL) muscles of rats submitted to 14 days of weightlessness produced by hindlimb suspension (HS). Ca 2+ uptake, Ca 2+ release and passive Ca 2+ leakage through the SR membrane were investigated using a method of caffeine-induced contracture on the single mechanically skinned fibers. In the SR of suspended soleus muscles, the rate of Ca 2+ uptake was higher than in the control muscles. However, there was no difference between the suspended and control muscles in the rate of Ca 2+ uptake of the SR in EDL after HS. In soleus muscles, Ca 2+ movements of the SR from the suspended muscle acquired the properties that were similar to those of the control fast muscle. The study of Ca 2+ leakage showed that the velocity and amount of passive Ca 2+ leakage from SR in soleus and EDL were apparently increased after HS. The results suggested that the functional properties of the SR membrane in slow and fast muscles were changed after HS.