Cross-correlation analysis of the naturally occurring discharges of .gamma.-motoneurons to gastrocnemius and soleus muscles has revealed a tendency towards synchronized firing in the decerebrated, spinal cat. The primary feature of the cross-correlation between 2 .gamma.-motoneurons is a symmetrical peak centered at time zero with reference to discharges at the level of the cell bodies in the ventral horn of the spinal cord. The peak has a half-width in the range 3-10 ms. Secondary features of the correlograms are frequently present, especially when the primary peak is large. They consist of dips to either side of the peak and are mirrored in the respective autocorrelation functions of the 2 contributory neurons. Apparently, they arise as a result of periodicity in firing patterns of the neurons. The degree of synchrony was measured as the ratio (k'') of the total counts contributing to the peak over the number expected by chance. The value of k'' was dependent upon the firing frequencies of the neurons. Plots of k'' against the geometric mean frequency of discharge showed a falling curvilinear relation as frequency increased. There was a weak inversion relation between the width of the cross-correlation peak and firing frequency. These relations had to be acknowledged before an assessment of change in the degree of synchrony could be made. The correlation between discharges is absent or weak in the decerebrated cat with an intact spinal cord and in intact cats anesthetized with pentobarbitone or chloralose. Anesthetics administered to a spinal animal did not suppress synchrony. The synchrony between .gamma.-motoneurons is characteristic of the short-term synchrony which arises from activity in shared presynaptic axons. In a few instances a stronger, more-tightly coupled synchrony was observed. The cross-correlogram peak had a half-width of 0.5-1.0 ms and was superimposed upon the usual wider-based peak. Evidently, this intense form of synchrony has a different origin. The properties of the short-term synchrony are discussed with respect to the mechanism of supraspinal control and a plausible model of the membrane properties of .gamma.-motoneurons which could account for the dependency of k'' on frequency of firing. Experiments to determine the afferent source of synchronous living and its distribution are presented in the following paper.