1. The ability of single dynamic fusimotor (γd) fibres to sustain the firing of muscle spindle primary (Ia) afferents during shortening was investigated in soleus muscles of anaesthetised cats. 2. Of 11 γd fibres, 10 could maintain Ia firing during 10 mm/s shortening. Of the 7 tested at greater velocities, 5 could maintain Ia firing during shortening at velocities greater than 50 mm/s. 3. This ability was, however, critically dependent upon the timing of the stimulation. In particular, it rapidly reduced with increasing duration of stimulation before the onset of shortening. Furthermore, if appreciable stretch occurred between the onset of γd stimulation and the onset of shortening, this could greatly reduce the ability of γd fibres to sustain Ia discharge. 4. If γd neurones are on occasion phasically activated during voluntary shortening movements, their action could be an important determinant of Ia firing, even in the presence of weak γs action. Therefore in chronic recordings, observation of Ia firing during muscle shortening is not an adequate criterion for inferring γd activity.