In the decerebrate cat preparation, microinjection of carbachol into the pontine reticular formation (PRF) induced muscular atonia by modulating the activities of reticulospinal neurons in the medullary reticular formation (MRF). The present study was designed to understand spinal interneuronal mechanisms involved in the control of postural muscle tone. For this purpose, extracellular activities of 105 interneurons were recorded from the lower lumbar segments (L6-L7) of acute decerebrate cats (n=28). Interneurons were divided into 3 types following inputs from the inhibitory region of the MRF and from peripheral sensory afferents. Type I cells (n=39) received excitation from the MRF and inhibition from volleys in flexion reflex afferents (FRA). They were located in the ventromedial part of the grey matter (lamina VII and VIII of the Rexed). Type II cells (n=38) received inhibition from the MRF. Type III cells (n=28) did not receive input from the MRF. Type II and III cells were located in the intermediate region (lamina IV-VII) and dorsal horn (lamina III) and exclusively received excitation from FRA. Firing rates of these groups of cells were further examined before and after injections carbachol into the PRF. In the state of decerebrate rigidity, Type I cells were silent or had low firing rates (2.0 ± 2.7 Hz; n=18). However, spontaneous firing was observed in Type II cells (38 ± 11.5 Hz; n=13) and Type III cells (21.4 ± 10.0 Hz; n=7). During carbachol-induced atonia, firing rates of the Type I cells was increased to 33 ± 11.7 Hz, while those of the Type II and III cells were reduced to 7.1 ± 5.2 Hz and 10.1 ± 5.8 Hz, respectively. These findings suggest the presence of functional topographical organization of spinal interneurons with respect to the control of postural muscle tone. Interneurons in the ventromedial grey matter (Type I cells) may be involved in muscular atonia, while those in dorsal horn and intermediate region (Type II and III cells) may contribute to maintenance of muscle tone.