The surgeon attempting to deal with uncontrollable pain by operative intervention is often in the unpleasant situation of not being certain whether the proposed intervention, which will interfere with the innervation of the involved region, will actually produce the desired effect over the entire painful area. It may happen, for example, with rhizotomy, where two neighbouring roots are both involved in the innervation of the painful area, that section of one root will denervate only part of the necessary field. Two neighbouring nerves may maintain a painful state in some diseases, for example, of the thoracic wall, and so on. The surgeon also wishes to know the effect of a surgical intervention in anterolateral chordotomy. If he could know this during the operation, he would be able immediately to relieve the patient of all pain, and spare him a second operation. Theoretically, it would be simple to carry out local anaesthesia in the area where tracts must be interrupted, then to test with stimulation and only then, under control, interrupt the sensory pathway from the skin. This is a procedure of a number of surgeons (Forster, Grant). Of course, every patient does not tolerate this, and some of those afflicted by pain demand total anaesthesia. Even if the patient agrees to such a procedure, not everyone is able to describe sensation precisely when tests are carried out on the skin during surgery. All of this complicates the value of this surgical aid. It has also been recommended that the patient should be anaesthetised only lightly, so that he can be easily aroused for sensory testing, and then the operation can continue under adequate anaesthesia once again. I do not know how this works out in practice, since I have only tried it, for other reasons, during a Sjoquist operation. Since, despite everything, it would appear that a satisfactory operative result is very dependent on determination of the extent of the painless area during operation, one must obviously choose a patient carefully for such collaboration.