Using cats lightly anesthetized with pentobarbital recordings were made of single muscle spindle discharge, muscle tensions, and the electroencephalogram. In the absence of muscle tension change, alterations in spindle discharge were attributable to supraspinal influences on the gamma motor neurons. Gamma efferent motor activity was transiently depressed, simultaneously in flexors and extensors, during spontaneous spindle bursts in pericruciate cortex as well as during spindle bursts triggered by single pulses to caudate and thalamic nuclei. Low frequency stimulation of caudate and thalamic nuclei, inducing recruiting and augmenting waves, was also accompanied by depression of gamma motor activity. The depressing effect was frequency-sensitive, reaching its maximum at stimulus frequencies approximating 0.5X and 1.0X the frequency of spontaneous spindle waves. The magnitude of the depression was also correlated with the amplitudes of spontaneous or triggered spindle bursts. The results were discussed from the veiwpoint that both spindle bursts and gamma motor depression were the expression of a change of activity in an unidentified locus.