We report the case of a head-injured patient with spontaneous hyperventilation who had recurrent episodes of relative hypoventilation associated with increases in intracranial pressure. Detailed ventilatory studies were performed during the 2nd week after injury. Our findings in this patient prompted us to review the possible mechanisms underlying the observed changes. We suggest that spontaneous hyperventilation in head injury is secondary to a decrease in cortical inhibitory influences on respiratory control mechanisms and that the transient episodes of relative hypoventilation observed in our patient may reflect modified ventilatory responses dependent on the altered state of consciousness. (Neurosurgery, 5: 701--707, 1979).