1. Mechanical properties of the surface membrane of skeletal muscle were determined on sarcolemmal vesicles (mean diameter, 71 microns) shed by rabbit psoas muscle swelling in 140 mM KC1 containing collagenase. 2. Vesicles were stressed by partial aspiration into parallel bore pipettes. The isotropic membrane tension so created caused an increase in membrane area which expresses itself in an elongation of the vesicle projection into the pipette. 3. For individual vesicles, a linear relationship between membrane tension and membrane area increase was found up to the point when the vesicle burst, i.e. sarcolemmal vesicles behaved as perfectly elastic structures. 4. The maximum tension sarcolemmal vesicles could sustain before bursting was 12.4 +/- 0.2 mN m-1 (median +/- 95% confidence interval), and the corresponding fractional increase in membrane area was 0.026 +/- 0.005 (median +/- 95% confidence interval). The elastic modulus of area expansion was 490 +/- 88 mN m-1 (mean +/- S.D.). 5. In conformity with cited comparable work on red blood cells and artificial lipid vesicles, the strength and area elasticity of the skeletal muscle membrane are considered properties of the fluid lipid matrix of the membrane and of the degree to which the bilayer is perturbed by lipid-protein interaction.