Single fibre group Ia excitatory post-synaptic potentials (e.p.s.p.s) were recorded in cat spinal motoneurones after the neurones were injected with tetraethylammonium (TEA) ions. TEA injection increased the peak amplitude of most e.p.s.p.s. The time course of e.p.s.p.s generated at the soma was unaffected, but the time course of e.p.s.p.s generated in the dendrites was prolonged. The membrane time constant did not change after TEA injection. Somatic e.p.s.p.s were voltage clamped after TEA was injected. The reversal potential for these e.p.s.p.s was more positive than for e.p.s.p.s unaffected by TEA. Composite e.p.s.p.s added linearly, or greater than linearly, whereas in motoneurones without TEA they added linearly or less than linearly. The enhanced amplitude and prolonged time course observed in dendritic e.p.s.p.s after TEA injection was reduced by small hyperpolarizing currents. Greater than linear summation of composite e.p.s.p.s was converted to linear summation by small hyperpolarizing currents. The increase in somatic e.p.s.p.s was attributed to a more positive reversal potential for the e.p.s.p.s. We suggest that TEA decreases the relative permeability of K+ in the subsynaptic channels. We propose that in the presence of TEA, dendritic depolarization activates an inward current which amplifies and prolongs synaptic potentials spreading towards the soma.