The study of pain is part of the research on the neurophysiological bases of sensation, since pain is merely a sensation of unpleasant feeling brought about by noxious stimuli. While the function of the other kinds of sensation is informative or gnostic, pain plays a protective role. The study of the physiology of pain includes the study of the receptors, the stimuli, the pathways, the structures of the central nervous system, the perception of pain and the motor and autonomic reaction. The receptors and the pathways of pain, as well as their specificity, are analyzed. The transmission of the quick and slow pain through different nervous paths until the cerebral cortex is discussed. The influence of attention on the degree of the perception of pain is commented. Based on experimental researches, the reticulo-cortico-reticular circuit is considered as indispensable for the perception of pain. Regarding the pathology of pain, the congenital analgesia is analyzed, the author accepting the hypothesis that in some synapses the pattern of impulses which give rise to the sensation of pain is changed. Emphasis 's given to the role played by the frontal lobe as part of a potentiating mechanism which conditions the general distress of the patient. Referred pain 's explained by the theory of "convergence-projection" of visceral fibers and cutaneous fibers for pain on the same neuron in some point of the sensory path. The relief or ceasing of pain brought abouth by hypophysectomy in cases of breast cancer gives rise to discussions on the role played by the hormones on the perception of pain.