1. A rapid imaging technique with temporal resolution of about 1 ms was employed to describe cell-length changes during the isotonic contraction cycle of adult rat ventricular myocytes at 22 degrees C. 2. Parameters of cell contraction and relaxation were defined and values obtained under control conditions and after treatment with 5 mumol/L verapamil. 3. Over 15 min, verapamil dramatically reduced the maximum shortening attained. This was associated with delay in both excitation-contraction coupling latency and the time at which maximum rate of shortening occurred. However, peak shortening was recorded earlier in the contraction cycle. Total cell-cycle time was abbreviated under the influence of verapamil and maximum rates of shortening and lengthening were depressed in a similar manner. 4. The results highlight the value of improved temporal precision in describing myocyte contractility and validate the use of the parameters defined in the single-cell model for the study of the mechanisms of action of cardiotonic agents.