The authors present two cases in which hyperventilation repeatedly led to hallucinations. They consider possible mechanisms and discuss their implications for the understanding of hallucinatory phenomena in schizophrenics, normals, and members of primitive societies. Four factors may be significant in determining whether hallucinatory phenomena will occur and in which sensory modalities: 1) the neurological integration of the individual; 2) the cerebral-vascular reactivity and pattern; 3) local cerebral requirements for oxygen and the capacity for extraction of it; and 4) the respiratory rate and buildup in hyperventilation.Abstract Teaser