Two subjects will be discussed: (i) the changes brought about in the central nervous system by a noxious input; (ii) certain states of consciousness in which human beings do not feel pain. Examples will be given of a pattern of neural activity being latent in the central nervous system after a period of localized pain. When this region is stimulated, even after a period of years, the previous pain is felt. Also, stimulation of the sensory cortex and thalamus in conscious patients shows permanent changes induced by constant pain. Studies from the literature are reported of human beings inducing trance states in themselves so as not to feel pain. In all of these states there is a great increase in sympathetic activity and an increase in the theta-activity in the electroencephalogram.