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Notes: Summary 1. The effect of stimulation of the red nucleus on interneurones in the dorsal horn and intermediate region in the lower lumbar spinal cord has been investigated in cats. It has been ascertained that the effects are mediated by the rubrospinal tract. 2. Extracellular monosynaptic focal potentials evoked by single volleys in the rubrospinal tract were recorded in Rexed's layer VI and VII from a region partly overlapping with that in which focal potentials from group I muscle afferents are evoked, but extending more ventrally. 3. Monosynaptic excitatory action from the rubrospinal tract (recorded in 60 of 340 interneurones) was found in two main categories of interneurones: a) cells monosynaptically activated or disynaptically inhibited from group I muscle afferents and b) cells di- or polysynaptically activated from the flexor reflex afferents or exclusively from cutaneous afferents. The cells under a) are located more dorsally than those under b) but both within the region in which rubral focal monosynaptic potentials are recorded. There was no evidence suggesting that rubrospinal fibres have monosynaptic connexions with interneurones not influenced from primary afferents. 4. Many of the group I interneurones in the intermediate region are without monosynaptic connexions from the rubrospinal tract as are the dorsal horn cells monosynaptically activated from cutaneous afferents and dorsally located cells which do not receive monosynaptic connexions from primary afferents but are polysynaptically activated from the FRA. 5. Late (di- or polysynaptic) excitatory, inhibitory or mixed postsynaptic rubral effects are common and were found in interneurones with or without monosynaptic connexions from primary afferents but receiving similar effects from the FRA. The occurrence of spatial facilitation between peripheral nerves and the rubrospinal tract in evoking late PSPs suggests that the late rubral PSPs are evoked by activation of interneurones transmitting actions from primary afferents. 6. Some consequences of the conjoint control of interneurones from primary afferents and the rubrospinal tract are discussed. The monosynaptic effects from the rubrospinal tract are considered in relation to the rubral control of Ib reflex pathways and to the disynaptic rubromotoneuronal PSPs evoked by monosynaptic activation of last order interneurones of polysynaptic reflex pathways from primary afferents. The late rubral effects on interneurones are discussed in relation to interactive mechanisms between segmental interneuronal pathways. Rubrospinal and corticospinal effects are compared. This work was supported by the Swedish Medical Research Council (Project No. 14X-9407C).