Experiment and clinical evidence of hypothalamic influence on autonomic function has accumulated since the early part of this century. Localization of hypothalamic lesions is difficult, and damage must be bilateral to produce symptoms. Caloric balance may be altered, with ventromedial lesions causing hyperphagia, and lateral hypothalamic lesions producing a syndrome of aphagia and weight loss in experimental animals. Gastric ulcerations are associated with brainstem lesions, including the diencephalon. Anterior hypothalamic lesions cause hyperthermia, while posterior lesions result in hypothermia, often accompanied by disturbance of sweating mechanisms. Disorders of sleep and wakefulness are seen clinically in encephalitis lethargica and Wernicke's syndrome, both associated with hypothalamic damage. The hypothalamus is the regulator and co-ordinator of central autonomic activity.