The pharmacological basis of the prolonged depressant effect of baclofen on small diameter afferent inputs to dorsal horn neurons was studied because of the possibility that this depression represents, at the cellular level, the mechanism of the analgesic effects of baclofen. When l-baclofen (0.1–3.0 mg/kg, i.v.) was given, the ensuing prolonged depression could not be mimicked or modified by the GABA-mimetic drug, muscimol (0.5–5.0 mg/kg, i.v.). Administration of the GABA antagonist, bicuculline (1–5 mg/kg, i.v.), partially reversed the depression induced by baclofen but even after large doses of bicuculline, subsequent administration of baclofen produced depression. The opiate antagonist naloxone (0.1–0.3 mg/kg, i.v.) and the glycine antagonist strychnine (0.1–0.2 mg/kg, i.v.) failed to block the prolonged depression induced by baclofen. These results suggest that baclofen is acting in the dorsal horn via a mechanism which excludes opiate receptors and glycine receptors and which only partially includes bicuculline-sensitive GABA receptors. The results are consistent with a possible action of baclofen on the recently proposed ‘novel GABA receptor’, or GABAB receptor, which is insensitive to bicuculline; if this indeed proves to be the case, then the present results suggest that at least in the spinal cord, these receptors are associated differentially with small diameter afferent fibres. However, regardless of the receptors involved, the most likely effect of baclofen is to reduce the amount of transmitter (or modulator) released from terminals of primary afferent fibres, and evidence is examined implicating glutamate and substance P. The transient, non-specific depressant effect (described in more detail in another publication) was prevented by bicuculline, and thus its effects are likely to be mediated by bicuculline-sensitive GABA receptors.