The effects of anabolic-androgenic steroid administration on the function of the sarcoplasmic reticulum (SR) pump were investigated in chemically skinned fibres from the extensor digitorum longus (EDL) and soleus muscles of sedentary rats. Twenty male rats were divided into two groups, one group received an intramuscular injection of nandrolone decanoate (15 mg kg−1) weekly for 8 weeks, the second received similar weekly doses of vehicle (sterile peanut oil). Compared with control muscles, nandrolone decanoate treatment reduced SR Ca2+ loading in EDL and soleus fibres by 49% and 29%, respectively. In control and treated muscles, the rate of Ca2+ leakage depended on the quantity of Ca2+ loaded. Furthermore, for similar SR Ca2+ contents, the Ca2+ leakage rate was not significantly modified by nandrolone decanoate treatment. Nandrolone decanoate treatment thus affects Ca 2+ uptake by the SR in a fibre-type dependent manner.