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1. The passive electrical properties of frog skeletal muscle fibres have been measured at a number of different sarcomere lengths (from 2-1 to 4-0 micron). The geometrical outline of each fibre was determined from optical cross-sections and sarcomere length was measured by laser beam diffraction. 2. When fibres were stretched to long sarcomere lengths the membrane capacity, Cm, of both normal and detubulated (glycerol-treated) fibres was significantly less than the Cm of fibres at rest length. A significant reduction in membrane conductance of fibres held at long sarcomere lengths was only seen with detubulated fibres. 3. Membrane capacity and membrane conductance have a significant dependence on the cross-sectional area of normal fibres but are independent of cross-sectional area after detubulation. 4. It has been shown that membrane geometry depends on the sarcomere length of the fibre and it is suggested that the passive membrane properties are related to sarcomere length because they depend on membrane geometry. 5. The specific membrane capacity, calculated from the data from detubulated fibres, is 0-8 micronF/cm2. 6. The internal resistivity, Ri, of normal fibres, also depends on sarcomere length between 2-1 and 3-0 micron. At a sarcomere length of 2-1 micron the average Ri is 122 +/- 3 omega. cm (mean +/- S.E. of mean) and at a sarcomere length of 3-0 micron the average Ri is 210 +/- 17 omega. cm (mean +/- S.E. of mean). No further increase in Ri was observed with further increases in sarcomere length.