Abstract: In single fibre electromyography it is useful to be able to measure intervals between waveforms (obtained from needle electrodes inserted into muscle fibres) reaching certain levels. A circuit is described, which is of general application, and which allows such intervals to be measured. First and second comparators which have adjustable reference levels receive an incoming waveform and, by means of polarity select circuits, provide outputs only when the reference levels are passed by waveform portions changing in selected respective directions. The time at which the first comparator is satisfied is available from a counter 33 but it is only passed to a computer interface when the second comparator has also been satisfied. Enable circuits prevent an output to the computer unless the comparators are satisfied within selectable time intervals.