A progressive decline in skeletal muscle function is part of the normal ageing process. The observed loss of specific force in aged muscle suggests that a Ca2+ dependent process may be impaired in ageing. The muscle force production is closely related to the amount of Ca2+ released from the sarcoplasmic reticulum (SR). For this reason, it would be important to have a reliable measurement of total SR Ca2+ content ([CaT]SR) in aged human skeletal muscle fibres under physiological resting conditions. The present study examined for the first time, in individual fibres from human skeletal muscle biopsies, whether endogenous SR Ca2+ content and maximal SR Ca2+ capacity are different between young and aged healthy adults. A muscle biopsy was taken from the vastus lateralis muscle from eleven and nine healthy young (23±0.8 yo) and old (70±0.7 yo) adults, respectively. After injection of a local anesthetic into the skin and fascia (1% lidocaine (Xylocaine)), a muscle sample was taken using a Bergstrom biopsy needle. Individual fibre segments, obtained from the biopsy, were mechanically skinned under paraffin oil so that they still contained their endogenous Ca2+ content. The total amount of Ca2+ contained in each fibre could be quantified by pre-equilibrating the fibre in a solution with a known concentration of the calcium-buffer BAPTA and then transferring the fibre to an emulsion of 1% Triton X-100 and paraffin oil (TX-oil) in order to lyse all membranous compartments and release any Ca2+ from within the fibre (Fryer & Stephenson, 1996). The total amount of Ca2+ present in the fibre can be calculated from the BAPTA concentration and the magnitude of the force response upon lysis. Furthermore, other fibre segments, prior to the TX-oil lysing, were loaded to their maximal SR Ca2+ capacity. Finally, using Western blotting, each muscle fibre was classified as type I or II according to the myosin heavy chain isoform present. When fibres with an endogenous Ca2+ content were assayed, the endogenous [CaT]SR obtained (expressed relative to intact fibre volume) was significantly decreased in aged subjects compared to young individuals (0.58±0.01 (n=15) and 0.67±0.03 (n=8) mmol.l-1, respectively, in type I fibres, and 0.62±0.02 (n=10) and 0.77±0.02 (n=15) mmol.l-1, respectively, in type II fibres). By loading the SR of the fibres maximally, the study also revealed that the maximal SR Ca2+ capacity was significantly decreased in fibres of aged subjects compared to young adults (1.22±0.03 (n=12) and 1.36±0.04 (n=14) mmol.l-1, respectively, in type I fibres, and 1.45±0.03 (n=11) and 1.72±0.03 (n=19) mmol.l-1, respectively, in type II fibres). The present results suggest that the appreciable decrease with ageing of the endogenous and maximal [CaT]SR in both type I and II fibres could be an important factor in muscle weakness in the elderly.