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A simple and inexpensive method for measuring the extent and rate of shortening of single myocytes was developed. The image of a single cardiac myocyte was focused through a 10x objective of an inverted microscope and projected onto a linear photodiode array having 256 self-scanning elements. This array was used as an edge detector to define cell length. The output from the array was fed into a digital-to-analog converter to produce an output voltage proportional to the myocyte length. In the present experiments the total scanning time for all the 256 elements was set at either 1.5 or 5 ms, thus allowing for detection of myocyte length changes during contraction. Results obtained using this system showed that the extent of shortening was 8.04 Â±0.48 percent of the resting cell length (n = 15). The rate of shortening in these cells was 99.49 Â±8.02 percent of the resting cell length/s. Similar results were obtained using a high speed film.