We studied the mechanism underlying acute organophosphate intoxication (OPI) through in-vivo and in-vitro electrophysiologic studies in rats injected with diisopropylfluorophosphate. Intoxicated rats showed weakness, repetitive compound muscle action potentials (CMAPs) in response to a single stimulus, and decremental response to repetitive nerve stimulation that was most pronounced at the second CMAP. The decrement was worsened with the administration of edrophonium, and was completely reversed by D-tubocurarine. In-vitro microelectrode studies showed no reduction in the amplitude of miniature endplate potentials (MEPPs) or in the quantal content of end-plate potentials (EPPs). However, the half-decay times of MEPPs and EPPs were significantly prolonged. Trains of stimuli induced sustained end-plate depolarization via a "staircase phenomenon" of summation of prolonged EPPs, which was enhanced by edrophonium and abolished by D-tubocurarine. These results indicate that sustained end-plate depolarization can directly account for the decrement and weakness in acute OPI.