The effect of parathion and para-oxon has been studied electrophysiologically using labellar hairs of the house fly, Musca domestica L., and the black blow fly, Phormia regina (Meigen). Direct applications of the insecticides were made in a glass capillary placed over individual sensory hairs. When applied directly, para-oxon produced a five-fold increase in frequency of spikes with the house fly; a frequency higher than controls continued for 25 minutes of a 30-minute period of study. Behavioral symptoms were studied following topical applications of the two organophosphates to the thorax of the flies. Symptoms produced were correlated with electrical responses recorded from labellar hairs of the affected flies. There was considerable distortion, variability of pattern, as well as increased frequency of spikes which occurred through the hyperexcitable period. Later, during knockdown, the frequency and pattern of spikes returned to conditions similar to controls. A short appraisal of the usefulness of the technique is given, particularly with respect to neurotoxins.