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1. Efferent intercostal alpha and gamma activity and afferent intercostal muscle spindle activity were studied in decerebrate cats in response to stimulation of the anterior lobe of the cerebellum and to postural and other reflexes.2. Low threshold intercostal responses were elicited from lobuli IV and V of the anterior lobe of the cerebellum.3. The existence of two functionally different types of intercostal gamma neurones has been confirmed. These are the ;rhythmic' or ;specifically respiratory' gamma neurones, and the ;tonic' gamma neurones.4. In response to cerebellar stimulation, facilitatory, inhibitory and diphasic tetanic and post-tetanic effects were obtained from alpha and the two types of gamma fibres in both external and internal intercostal nerve branches.5. Generally both inspiratory and expiratory alpha and gamma activity was facilitated in response to tetanic stimulation at contralateral stimulus sites, and inhibited in response to stimulation of ipsilateral sites.6. ;Rhythmic' gamma activity appeared to be rather closely linked to the respiratory alpha activity but the balance between ;rhythmic' gamma and alpha was often changed in response to cerebellar stimulation, as indicated by the responses of primary muscle spindle afferents.7. The ;tonic' gamma neurones were as a rule more responsive to cerebellar stimulation than were the alpha and ;rhythmic' gamma neurones. Long-lasting post-tetanic effects were much more prominent in the ;tonic' gamma fibres than in the alpha or ;rhythmic' gamma fibres.8. ;Rhythmic' gamma activity was abolished after cervical transections of the cord. ;Tonic' gamma activity remained in the spinal preparations although usually at a different discharge rate.9. ;Tonic' gamma neurones were more responsive than the ;rhythmic' gamma neurones to the proprioceptive gamma reflex elicited by passive movements of the chest wall as well as to other spinal and supraspinal reflexes.10. Both ;dynamic' and ;static' gamma fibres seem to be represented in the group of ;tonic' intercostal gamma neurones.11. The significance of the results are discussed with regard to the following two main points: (a) integration of respiratory and postural functions of the intercostal muscles; (b) cerebellar control of the alpha-gamma balance in active contractions and passive relaxations.