In the cat, stimulation of group II afferents from hindlimb muscles evokes different crossed reflex actions depending on the integrity of the spinal cord: with the cord intact, extensor motoneurons are inhibited by activation of contralateral group II afferents; after spinal transection, the same stimuli excite these neurons (crossed extension reflex). We have investigated the mechanisms underlying this descending control. To delimit the descending pathway, the effects of funicular lesions of the thoracic cord on the crossed actions on motoneurons were examined. Bilateral lesions of the dorsolateral funiculi abolished the crossed IPSPs as effectively as complete spinal section. If either dorsolateral funiculus was spared, the IPSPs remained. To examine whether serotonergic fibers were involved, the effects of agents selective for 5-hydroxytryptamine (5-HT)1A receptors were examined. After abolishing the crossed IPSPs by spinal transection, systemic administration of the 5-HT1A receptor agonist (+/-)-8-hydroxy-2-(di-n-propylamino)tetralin hydrobromide (8-OH-DPAT; 0.1-1.3 mg/kg, i.v.) restored the crossed inhibition. This effect was antagonized by the selective 5-HT1A receptor antagonist WAY-100135 (0.7-3.7 mg/kg, i.v.). The simplest explanation of these findings is that a serotonergic pathway, descending via the dorsolateral funiculi and acting via 5-HT1A receptors, is involved: with the spinal cord intact, the pathway would be tonically active and thus enable the crossed inhibition. This raises the possibility that a serotonergic pathway is involved in the selection of specific spinal reflex patterns via 5-HT1A receptors.