This review attempts to summarize: (i) evidence on how man voluntarily or behaviourally (as in speech) alters breathing; and (ii) evidence on how the breathlessness induced by CO2 inhalation, is perceived. The application of new methods to study these problems, e.g. functional brain imaging and transcranial focal brain stimulation, is summarized. Studies of patients with specific neurological lesions have shed considerable light in this area. The key requirement for the ponto-medullary respiratory oscillator to be both 'intact' and 'responsive' for the perception of CO2-induced air hunger is emphasized. We are ignorant as to how the voluntary/behavioural control system interacts with the automatic system at any site above the final common pathway of the respiratory anterior horn cells in the cervical and thoracic spinal cord. The opportunities for further work are outlined.