The literature on the classical mechanical properties of geometrically simple mammalian smooth muscle preparations is analyzed in an attempt to characterize the contractile system. Although the physiological behavior of smooth muscle is diverse, indices of contractile system function suggest there is considerable uniformity at this level. Compared to vertebrate striated muscles, smooth muscle tissues and possibly individual cells (i) often develop equal or greater maximum isometric force/cell cross-sectional area, with (ii) low and fairly uniform maximum shortening velocities, and (iii) operate over a range of lengths which may be no greater than the potential in striated muscle. Few details are known of filament structure, organization, interactions, and of force transmission in comparison with the extensive information on the contractile unit (sarcomere) for striated muscle. Consequently, tissue or cellular mechanical parameters may not be directly proportional to the output of the myofilamental contractile unit in smooth muscle. A sliding filament mechanism involving actin-myosin interactions qualitatively similar to those in striated muscle seems probable. However, there are important quantitative differences in the contractile apparatus which may be related to functional properties.