1. Monosynaptic excitatory post-synaptic potentials (EPSPs) produced in spinal motoneurones of the cat by stimulation of a single afferent fibre were recorded with intracellular electrodes.2. In total, seventy-three triceps surae motoneurones were studied with stimulation of thirty-six different afferent fibres.3. The mean amplitude of the EPSPs evoked by single afferent impulses ranged from 0.06 to 2.0 mV with an average of 0.27 mV.4. The mean number of unit EPSPs responding to a single afferent impulse (m) was calculated from the number of failures. The values ranged from 0.7 to more than 5. About 10% of the sample showed no failure of synaptic response in about 200 consecutive trials. The m values for these synaptic responses were estimated to range from 5 to 15.5. In the majority of tests, the observed amplitude fluctuations of monosynaptic EPSPs evoked by stimulation of a single fibre were less than those expected from Poisson's law. This discrepancy may be accounted for by non-linear summation of the unit EPSPs at dendritic synaptic sites.6. It is suggested that the synaptic responses initiated at different sites of a motoneurone may summate linearly at the soma, although summation of unit EPSPs is non-linear at individual synaptic sites.