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A quantitative analysis of the volumes, surface areas, and dimensions of the ultrastructural components in the soleus muscle fibers of the guinea pig was made by using point counting methods of stereology. Muscle fibers have structural orientation (anisotropy) and have spatial gradients of the structures within the fiber; therefore the standard stereological methods were modified where necessary. The entire analysis was repeated at two section orientations to test the modifications and identical results obtained from both. The volume of lipid droplets was 0.20 +/- 0.06% (mean +/- standard error, n = 5 animals) and the nuclei volume was 0.86 +/- 0.20% of the fiber volume. The total mitochondrial volume was 4.85 +/- 0.66% of the fiber volume with about one-third being found in an annulus within 1 microm of the sarcolemma. The mitochondrial volume in the remaining core of the fiber was 3.6 +/- 0.4%. The T system has a volume of 0.14 +/- 0.01% and a surface area of 0.064 +/- 0.005 microm(2)/microm(3) of the fiber volume. The surface area of the sarcolemma is 0.116 +/- 0.013 microm(2)/microm(3) which is twice the T system surface area. The volume of the entire sarcoplasmic reticulum is 3.52 +/- 0.33% and the surface area is 0.97 +/- 0.09 microm(2)/microm(3). The sarcoplasmic reticulum is composed of the terminal cisternae whose volume is 1.04 +/- 0.19% and surface area is 0.24 +/- 0.05 microm(2)/microm(3). The tubules of the sarcoplasmic reticulum in the I band and A band have volumes of 1.97 +/- 0.24% and 0.51 +/- 0.08%, and the surface areas of the I and A band reticulum are 0.56 +/- 0.07 microm(2)/microm(3) and 0.16 +/- 0.04 microm(2)/microm(3), respectively. The Z line width, myofibril and fiber diameters were measured.