1. Systematic studies of the response of baboon's tibialis anticus muscle spindles to stretch were undertaken. Most of the spindle afferents studied had conduction velocities between 72 and 78 m/sec with a range from 39 to 93 m/sec. There was no clear bimodality in the histogram of the conduction velocity. Measurements were made of the axon diameters of the nerve to tibialis anticus. The largest number of the larger axons had diameters from 11 to 13 mu. The largest axon diameter measured 17 mu and there were very few of these.2. The dynamic index for any given afferent tended to be greater for the more rapidly conducting afferents and lower for the more slowly conducting afferents. However, a statistically significant regression line of this relationship could only be drawn for a stretch velocity of 64 mm/sec.3. The spindle afferent response to stretch was studied from different muscle lengths. It was found that the velocity sensitive portion of the response to phasic stretch decreased when the stretch extended up to or beyond the maximum physiological length of the muscle.4. Dynamic and static fusimotor fibres were isolated. The response of the spindle afferent to stretch, while the dynamic fusimotor fibre was being stimulated, was the same as that reported for the cat by P. B. C. Matthews (1962). During static fusimotor stimulation the response of the spindle afferent to stretch was usually like that reported for the cat. In a single case, however, static fusimotor stimulation resulted in a lowering of the peak frequency of the response of the spindle afferent to the phasic portion of stretch.