When pain occurs in teeth as a result of external stimulation it is more likely to be due to physical factors affecting the enamel and dentine than to pulp disease. The mechanisms are described, clinical conditions are mentioned and a speculative attempt is made to correlate them by introducing the idea of streaming potentials in the dentine and pulp. The pain perception threshold is considered in relation to different teeth, age, sex, lateral dominance, personality, and the area of electrode contact. The latter has proved interesting, for manipulation of it has led to possibly important findings on the nature of convergence in the trigeminal system, two-point discrimination, referred pain and somatopic representation of dental pulps in the medulla oblongata.