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The design and characteristics of a video-based device that can noninvasively measure the extent and rate of shortening of isolated cardiac myocytes are discussed. Construction of the motion detector is relatively inexpensive. It is easy to use because edge selection is simple and the video images of contracting cells can be analyzed at the experimenter's convenience. The device also exhibits high spatial and adequate temporal resolution. The physiologic application of the motion detector was examined by measuring overall shortening in enzymatically dispersed adult guinea pig and rabbit cardiac myocytes bathed in Tyrode's solution containing 2.7 mm calcium at 36 degrees C (pH 0 7.4). When stimulated at 0.5 Hz, myocytes shortened an average of 8.1% (guinea pig) and 9.0% (rabbit) of cell length. respectively. The corresponding mean maximum rates of shortening were 81.7% (guinea pig) and 97.5% (rabbit) of cell length/s, respectively. The motion detector was also used to measure shortening between closely spaced markers on the cell surface.