A new method is described for estimating the shape of the compound post-synaptic potentials evoked by stimuli in human motoneurones. The method is based on changes in the duration of the interspike intervals in motor-unit spike trains that are time-locked to the stimulus. This is particularly helpful in estimating the profile of long-latency slow rise-time post-synaptic potentials that are difficult to estimate with other methods. The method is simple to apply and the analysis is readily implemented on a personal computer.