SUMMARY1. Spontaneous transmitter release, recorded as miniature end-plate potentials (m.e.p.p.s), was studied in rat extensor digitorum longus (e.d.l.) and soleus muscles partially or completely paralysed by botulinum toxin type A (BoTx). Normal unpoisoned muscles were examined for comparison.2. Analysis of m.e.p.p.s in both normal and BoTx-poisoned muscles confirmed the presence of two populations of potentials. One population, which comprised about 96 % of the m.e.p.p.s recorded at non-poisoned end-plates, was characterized by a uniform time course and a mean time-to-peak of 0 5-0-7 ms. These potentials had a shape and time-to-peak similar to that of quantal end-plate potentials (e.p.p.s) evoked by nerve stimuli. These were designated 'fast m.e.p.p.s'. The other population of m.e.p.p.s was characterized by a slower, more variable rise-time, the time-to-peak exceeding 141 ms, and generally a larger amplitude. These were designated 'slow m.e.p.p.s'.3. In both partial and complete paralysis by BoTx the frequency of fast m.e.p.p.s was reduced by more than 90 % and the reduction lasted several weeks. After 6-10
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