The excitabilities of separate fibre populations were examined in the ulnar nerve in 74 healthy subjects, and in the posterior tibial, peroneal, and sural nerves in 23 of these subjects. Square wave electric stimuli of varying duration were applied to the nerve in different locations, while recording the evoked muscle or nerve action potential. "Threshold" strengthduration curves were derived for motor, sensory, and mixed nerve fibre populations. These curves did not vary significantly one from another, in upper versus extremity, or over time. No correlations between excitability and other measures of peripheral nerve function were observed. Excitability appears to reflect some element of nerve structure or function other than myelin.