The question of receptor specificity was examined in isolated Pacinian corpuscles. The receptor membrane of this mechano-sensitive organ is located at the non-myelinated nerve ending inside the corpuscle. The nerve ending was isolated by dissection from the corpuscle and subjected to mechanical and thermal stimulation. The following results were obtained. The nerve ending is rather specifically sensitive to mechanical stimulation. Mechanical stimuli that compress or displace the nerve ending by 0.2-0.4 [mu] produce excitation[long dash]namely, generator potentials in its membrane. Thermal stimuli fail to produce any response from it. The well-organized tissue of the corpuscle that surrounds the nerve ending plays no part in this specific sensitivity. The high degree of mechano-sensitivity is confined to the length of about 700 [mu] of non-myelinated nerve ending which is the receptor proper. The adjacent first node of Ranvier, as well as all other regions of the myelinated axon of the sense organ, is relatively insensitive to mechanical stimulation.