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Excitatory post-synaptic potentials (e.p.s.p.s) were evoked in motoneurones of anaesthetized cats by impulses in single group 1 a axons. E.p.s.p.s with a time course which indicated a somatic site of origin were voltage-clamped using a single micro-electrode clamp. Excitatory post-synaptic currents (e.p.s.c.s) were found to peak in less than 0.2 ms, and to decay with an exponential time course. The time constant of decay was usually in the range 0.3-0.4 ms (at 37 degrees C). At the resting membrane potential, an e.p.s.p. with a peak of 100 microV was generated by an average peak e.p.s.c. of 330 pA. This corresponded to an average peak conductance increase of 5 nS. The e.p.s.c. decreased with membrane depolarization, and reversed to become an outward current at a null potential of +4.6 +/- 2 mV (+/- S.E. of mean; n = 7). Membrane hyperpolarization caused the peak e.p.s.c. to increase and the time constant of decay of the e.p.s.c. to decrease. The total charge in the synaptic current did not increase with hyperpolarization. This observation can explain earlier observations which showed that the peak amplitude of the e.p.s.p. did not increase with hyperpolarization. The number of ion channels opened by transmitter release at a single somatic bouton was estimated to be in the range 40-240.

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