The relationship between the depression in the amplitude of the compound muscle action potential and neuromuscular decrement (fade) was studied during the induction of non-depolarizing blockade, using a train of four supramaximal stimuli. Neuromuscular decrement (%) was defined as: [1 - (amplitude of fourth muscle action potential)/(amplitude of first muscle action potential)] x 100. When the amplitude of the first action potential was reduced by 50%, mean neuromuscular decrement increased in the order pancuronium < alcuronium < tubocurarine < frazadinium < gallamine. Similarly, the slope of the regression line relating the decrease in the amplitude of the action potential to decrement was least with pancuronium and greatest with gallamine. These results may reflect different affinities or intrinsic activities of the five drugs for prejunctional and postjunctional receptors. Thus, pancuronium may have a greater affinity for postsynaptic receptors, while tubocurarine and gallamine affect selectively the motor nerve terminal. It was confirmed that fazadinium had a more rapid onset on action than any of the other myoneural blocking drugs studied.