Electrical excitation of tooth pulp afferents seems to be a most accurate and reproducable method to produce graded pain with minimal injury of the tissues. Besides electrical current only thermal stimulation may activate pulp nerves of an intact tooth, but this method lacks the precision of the electrical stimulation and bears more risk of injury. Surrounded by enamel of high resistance and capasitance the pulp nerves are difficult stimulation objects. On the other hand with careful stimulation the current is almost completely restricted to the pulp. The square wave current pulse is "deformed" according to the time constant of hard tissues and exceptionally long durations are needed. There is ultimate necessity for constant current stimulation because of varying anatomy and resistance of the teeth. When different amplitudes and durations of current pulses are used to compare perception thresholds of human teeth and firing thresholds of single pulp nerve fibres of the cat, it may be concluded that "prepain" sensation is experienced using current values that activate only A-fibres. To attain the pain threshold, recruitment of more A-delta fibres with increasing current is needed, but the lowest thresholds of C-fibres are even higher.