Self-reported affective and somatic disturbances and heart rate changes resulting from a brief period of voluntary hyperventilation are presented and related to individuals' Eysenck Personality Questionnaire (E.P.Q.) scores. Considerable individual variability was observed in the effects of hyperventilation. Neuroticism was significantly correlated with affective but not self-reported somatic or objectively measured heart rate changes. Other possible determinants of observed variability and its possible relevance to the etiology of panic attacks are discussed.