Occupational exposure to fenvalerate, a synthetic pyrethroid insecticide, has been reported to cause paresthesia. An assay was devised in our laboratory for subjective grading of the sensation produced by the topical application of this compound. The present double-blind study compared human discrimination of topically applied technical fenvalerate, the heavy-ends fraction of fenvalerate, and ethyl alcohol (vehicle). Both forms of fenvalerate showed a statistically significant increase in inducing paresthesia over the vehicle alone. The onset of the cutaneous sensations occurred at one hour, peaked at three to six hours, and lasted approximately 24 hours. Numbness, itching, burning, tingling, and warmth were the most frequently reported sensations. The difference between the two fractions of fenvalerate was not statistically significant.