Successive stimuli of constant intensity applied to Ia afferents produce spinal monosynaptic reflexes (MSRs) of variable amplitude. We recorded simultaneous MSRs in the left and right L7 (or L6) ventral roots of anaesthetized cats. We analysed the cross-covariance between the amplitudes of bilateral MSRs. Long time series (5 to 8 hours) of these bilateral MSRs exhibited transitory changes in their covariations (as measured by the zero-lag peak of their cross-covariance), thus suggesting the existence of certain neural sources contributing to produce these changes. The aim of the present study was to show that spinal centers producing negative spontaneous cord dorsum potentials (nSCDPs) contribute to maintain correlations in the amplitude of bilateral MSRs. After spinal cord transection at the L1 segment, no significant changes were observed in the correlation between the amplitude of bilateral nSDCPs versus the amplitude of bilateral MSRs. However, this correlation, as well as the peak at zero lag in the cross-covariance between bilateral MSRs, and the cross-covariance between bilateral nSCDPs, respectively, were abolished after a subsequent longitudinal bisection at the L1-S2 spinal segments. These results suggest that lumbar spinal neurons (bilaterally interconnected) contribute to maintain the synchronous fluctuations of bilateral MSRs.