Smooth muscle's slow, economical contractions may relate to the kinetics of the crossbridge cycle. We characterized the crossbridge cycle in smooth muscle by studying tension recovery in response to a small, rapid length change (i.e., tension transients) in single smooth muscle cells from the toad stomach (Bufo mar/nus). To confirm that these tension transients reflect crossbridge kinetics, we examined the effect of lowering cell temperature on the tension transient time course. Once this was confirmed, cells were exposed to low extracellular calcium ([Ca~+]o) to determine whether modulation of the cell's shortening velocity by changes in [Ca2+]o reflected the calcium sensitivity of one or more steps in the crossbridge cycle. Single smooth muscle cells were tied between an ultrasensitive force transducer and length displacement device after equilibration in temperature-controlled physiological saline having either a low (0.18 mM) or normal (1.8 mM) calcium concentration. At the peak of isometric force, after electrical stimulation, small, rapid (_<1.8% cell length in 3.6 ms) step stretches and releases were imposed. At room temperature (200C) in normal [Ca~+]o, tension recovery after the length step was described by the sum of two exponentials with rates of 40-90 s -t for the fast phase and 2-4 s -I for the slow phase. In normal [Ca2+]o but at low temperature (10~ the fast tension recovery phase slowed (apparent Oo0 = 1.9) for both stretches and releases whereas the slow tension recovery phase for a release was only moderately affected (apparent 0o0 = 1.4) while unaffected for a stretch. Dynamic stiffness was determined throughout the time course of the tension transient to help correlate the tension transient phases with specific step(s) in the crossbridge cycle. The dissociation of tension and stiffness, during the fast tension recovery phase after a release, was interpreted as evidence that this recovery phase resulted from both the transition of crossbridges from a low-to high-force producing state as well as a transient detachment of crossbridges. From the temperature studies and dynamic stiffness measurements, the slow tension recovery phase most likely reflects the overall rate of crossbridge cycling. From the tension transient studies, it appears that crossbridges cycle slower and have a longer duty cycle in smooth muscle. In low [Ca2+]o at 20~ little effect was observed on the form or time course of the tension transients. These data may indicate that the calcium dependence of the maximum shortening velocity (VmJ (Warshaw, D. M., S. S. Work, and W. J. McBride. 1987b. Pfliigers Archiv. 410:185-191) reflects the calcium sensitivity for one or more steps in the crossbridge cycle that are rate limiting for Vm~ but are not expressed in tension transients. An alternative explanation is the existence of an internal load within the cell that impedes crossbridge cycling as the cell shortens.