The electrocardiograph has eight input channels and the signals are multiplexed, digitised and stored using a microprocessor system, The 12 routine leads data are the computed and plotted by a fast microdot thermal printer/plotter. Digital filtering for EMG signals can be used and an extremely effective 50 Hz interference elimination digital filter is provided, simplifying the design of the input circuitry and avoiding the use of an earth connection. Thus 12 synchronous leads, plotted sequentially, are obtained. The necessary lead designations, filter data and error messages are automatically printed out together with the electrocardiogram. The operation includes simple pushbutton selection of one of three modes (12 leads or peripheral leads—3 s epoch each and a ll-lead 30s epoch) and the activation of a ‘start’ key