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1. Studies have been made of the anatomical localization in the brain stem of the sites at which tetanic stimulation evoke inspiratory and expiratory apneusis.

2. The inspiratory responses arise from a relatively circumscribed region within the medulla corresponding to the nucleus reticularis giganto-cellularis and ventralis which give rise to the medullary contingent of the long reticulo-spinal tracts. Expiratory responses were obtained dorsal and lateral to this area, but not localized to any cyto-architectonically distinct region of the reticular formation.

3. During the apneustic responses there was co-activation of the intercostal alpha and fusimotor neurones with reciprocal inhibition of the antagonistic motoneurones. The threshold for activation of the fusimotor neurones was usually lower than for the alpha motoneurones.

4. Results with brief tetanic stimulation suggest that the long reticulospinal tracts are responsible for the apneustic responses and that the effects are mediated at segmental level over an interneuronal pathway.

5. The response of the intercostal motoneurones during the apneustic responses is shown to be dependent on the integrity of the dorsal spinal roots.