Semitendinosus fibres from Rana pipiens were examined in the electron microscope. When the aqueous solutions in which the fibres were prepared contained no sucrose, the sarcoplasmic reticulum was markedly swollen and possibly fragmented. When as little as 50 mM sucrose was included in the aqueous solution, the sarcoplasmic reticulum retained the flattened appearance characteristic of intact fibres. The degree of flattening was not substantially different with concentrations of sucrose ranging from 50 to 300 mM, suggesting that the flattened volume may be determined by structural elements. In all the fibre segments, the regularity of the hexagonal filament lattice was disrupted and the disruption was not affected by sucrose. Radioactive calcium uptake was measured in the presence and absence of 200 mM sucrose. The capacity of the sarcoplasmic reticulum (SR) was depressed by about 30% by the sucrose but the initial uptake was not significantly affected. This finding suggests that most of the calcium within the SR is bound and not free in solution.