SUMMARY1. The amplitudes of end-plate currents (EPCs) in short trains of fifteen to seventeen EPCs at 10 Hz were depressed in the presence of 10 ,uM-proadifen when acetylcholinesterase (AChE) was inhibited.2. The proadifen-induced EPC depression was voltage-dependent and the effect was more pronounced at negative membrane potentials.3. In the presence of proadifen, the mean amplitude of miniature end-plate currents (MEPCs) was reduced by 36% 5 s after the EPC train as compared with MEPCs before the train.4. Without proadifen, but with inhibited AChE, an increase of temperature from 20 to 26°C and elevation of external Ca2+ from 1-8 to 2-5 mm led to EPC amplitude depression in the train, which was also potential-dependent.5. After AChE inhibition, proadifen (10,uM) progressively shortened MEPC decay without significant reduction of amplitude up to 40 min of exposition. MEPCs were not affected by proadifen when AChE was active.6. It is concluded that these postsynaptic effects of proadifen can be explained neither by its action on the resting acetylcholine receptors (AChR) nor on open ion channels but are due to its desensitization-promoting action.