The following mechanical tests have been performed on spontaneously contracting taenia coli: isometric step stretch, isotonic step loading, and length-tension. Passive stress-strain tests have also been done. Information on spontaneous contraction was more readily obtained in isometric tests. In length-tension tests, each isometric contraction was analyzed on a phase plane which is a representation of muscle stress, sigma(t), versus the rate of change of stress, sigma(t). The curve in the phase plane provided parameters for describing spontaneous contraction as a function of muscle length, L. The maximum active stress in a cycle of spontaneous contraction, S(L)max, and the maximum rate of increasing active stress, [S(L)]max, were shown to depend on L in the same way. Both parameters attain a maximum value (upper bound) at the same muscle length, Lmax. When L less than Lmax, the minimum stress in a spontaneous contraction, sigma(l)min, is negligible. If L greater than Lmax, sigma(L)min increases nonlinearly and is equal to the passive stress, tau(L), in an epinephrine-relaxed muscle. The results show that the ratio of [S(L)]max to S(L)max, eta(L), is constant for a physiological range of muscle length. This suggests that eta(L) may be used as an index of contractility for taenia coli.