The displacement cardiograph (d.c.g.) is a noncontacting electromagnetic transducer that can be used to measure movement of tissue. In normal operation a sensing probe is placed near the moving tissue, and this produces an output voltage. The d.c.g. detects movement by means of the electromagnetic field it generates. Since this field penetrates into tissue, two sources of output exist: changes in the air-gap distance between the tissue surface and the probe, and movements occurring within the tissue itself. The dominant source of signal proved to be movement of the tissue surface. A quantitative relationship exists between instrument output and surface movement only when the movement is primarily one-dimensional. Where surface movement is small or absent, instrument response is due to movements within the tissue. The significance of these results is discussed in relation to measurement of heart movements in the closed and open chest.