The present study aimed at determining the role of the serotonin(1A) (5-HT(1A)) receptor subtype on the modulation of the mono-synaptic reflex (MSR) elicited by dorsal root stimulation and recorded from ventral roots in the hemisected spinal cord obtained from rat pups. Serotonin and 5-carboxamidotryptamine (5-CT) depressed both the MSR and the cumulative depolarisation (CD) produced by repetitive dorsal root stimulation, whereas the specific 5-HT(1A) receptor agonist (R)-(+)-8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) applied at 1 microM showed a selective depressant effect on the MSR. Superfusion of the 5-HT(1A) receptor antagonist (+)-N-tert-butyl-3(4(2-methoxyphenyl)-piperazin-1-yl)-2-phenylprpanamide ((+)WAY 100135) produced a complete blockade of 8-OH-DPAT effects. In addition (+)WAY 100135 blocked partially the effects of 5-HT and 5-CT on the MSR but not the effects of these compounds on the CD. The results are consistent with an intervention of 5-HT(1A) receptors in the modulation of non-nociceptive reflexes but do not support a prominent role for this receptor in the modulation of nociceptive reflexes.