The discharge activity of single muscle spindle receptors was recorded in freely moving cats. Large changes in responsiveness to length variations was observed in different types of movement. In separate stimulations on anaesthetised cats, the activity of the gamma-fusimotor neurones responsible for these changes was reconstructed. The results suggested that fusimotor action on a given spindle afferent during the movements studied was not rigidly alpha-linked, but 'set' by the CNS to steady levels, and that it could switch from largely static (gamma s) to largely dynamic (gamma d) according to the motor tasks performed.