A quantitative study of muscle cells of the guinea-pig taenia coli is reported. Stereological methods were used on electron micrographs and phase contrast micrographs. Smooth muscle cells of taeniae fixed under 1 gram load were about 515 mum long. Muscle cell volume was about 3,500 mum3 and cell surface 5,300 mum2. About 168,000 caveolae were found at the surface of each muscle cell, covering about 29% of its surface. They produced a 73% increase of the cell membrane compared to a smooth-surfaced cell. The ratio surface-to-volume is about 1:0.67 if the geometrical surface is considered, or 1:0.39 if the total surface of the cell membrane (including the caveolae) is considered. Mitochondria constituted 3.5-4% of the cell volume. A few nexuses were observed, both between two muscle cells and between a muscle cell and an interstitial cell. In serial sections septa of connective tissue and groups of muscle cells were found to disappear within few tens of microns or to merge with other septa, and the taenia did not appear to be divided into clear-cut muscle cell bundles. Bundles of smooth muscle cells were seen passing from the taenia to the underlying circular muscle. The transverse sectional area of the taenia ranged between 0.14 and 0.39 mm2; it showed about 526 blood vessels-mm-2.