In the unanesthetized spinal cord of the cat, simultaneous intracellular recordings were made from two motoneurons belonging to the gastronemius motor nucleus. 2. Supramaximal iterative stimulation of small branches of the gastrocnemius nerve produced monosynaptic EPSPs (Ia EPSPs) of varying amplitude superimposed on a fluctuating base line. 3. In most cases the variance of the motoneuron membrane potential was increased above base-line levels with a time course approximately matching the Ia EPSP. This suggests that Ia EPSP fluctuations are greater than can be accounted for by the base-line fluctuations alone. 4. For a given series of Ia EPSPs, the smaller responses in the series had about the same decay phase as the larger EPSPs, suggesting that most of the Ia EPSP fluctuations were not due to systematic changes in postsynaptic conductances produced by ongoing activity, but rather to a presynaptic mechanism. 5. Simultaneous recording from two motoneurons showed that base-line fluctuations were positively correlated. In most cases, however, there was an additional increased correlation above base-line levels resembling the time course of the Ia EPSPs, indicating positive correlation between EPSP fluctuations which is attributed to a presynaptic mechanism. 6. Conditioning volleys to group I muscle afferents or to low-threshold cutaneous afferents reduced the variance of the Ia EPSPs and also their correlation in motoneuron pairs, often without changing the mean Ia EPSPs. 7. It is concluded that, in the unanesthetized spinal cord, in addition to the random process which governs transmitter release intrinsic to a given synaptic terminal, there is another stochastic process affecting, in a correlated manner, transmitter release in large sets of Ia synaptic terminals. Most likely, the correlation in transmitter release is achieved by membrane potential fluctuations imposed on the Ia terminal arborizations by ongoing activity of the segmental mechanism mediating primary afferent depolarization. 8. The effects of such a correlating influence on cell firing behavior have been analyzed. The results suggest that this mechanism, referred to as control by presynaptic correlation, is able to modulate the information transmitted from Ia fibers to motoneurons.
