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1. Excitatory post-synaptic potentials (e.p.s.p.s) were evoked in spinal alpha-motoneurones of the cat by impulses in single group Ia nerve fibres. 2. The average peak amplitude of some of these e.p.s.p.s was increased by a conditioning tetanus. The maximum increase observed was 54% of the control amplitude. 3. The average peak amplitude of some e.p.s.p.s was increased by a single conditioning stimulus which preceded the test stimulus by 1 or 2 msec. The maximum increase observed was 28% of the control amplitude. 4. The ability of e.p.s.p.s to potentiate following a tetanus was correlated with their ability to be facilitated by a single conditioning stimulus. 5. If an e.p.s.p. could be facilitated prior to a tetanus, the amount of facilitation was reduced after the tetanus, with all facilitation being abolished when post-tetanic potentiation was maximal. 6. The fluctuations of an e.p.s.p. were analysed before and after a tetanus. The peak amplitudes that an e.p.s.p. fluctuated between while potentiated did not gradually diminish as the effect of the tetanus disappeared. Post-tetanic potentiation, when it occurred, was accompanied by a decrease in the probability of occurrence of components with smaller peak amplitudes and an increase in the probability of occurrence of components with larger peak amplitudes. 7. These results are consistent with the suggestion that the magnitude of the synaptic potential generated at a single bouton does not vary from trial to trial (Jack, Redman & Wong, 1981a). Nor does the amplitude of this potential vary following a single conditioning stimulus or a tetanus. Post-tetanic potentiation and facilitation result from a decrease in the probability of failure to release transmitter following the conditioning stimuli.

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