A novel stimulator was developed to examine the contribution of diaphragmatic failure to the toxicity of cholinesterase inhibitors. The hardware consisted of a laboratory computer equipped with an analog input/output board. The pulses were generated by a computer program written in Fortran-77 and assembly language and were amplified to the range required for nerve by an operational amplifier in a voltage follower configuration. The salient feature of the stimulator was its ability to produce diaphragmatic contractions where increases in firing frequency and motor unit recruitment occur simultaneously during each inspiratory cycle. This method has been designated as stimulation to distinguish it from conventional repetitive where pulse amplitudes and frequencies are fixed for each train. When tested on in vitro preparations of rat diaphragm muscle during physostigmine-induced cholinesterase blockade, both conventional and contour led to marked tetanic fade. How...