To determine whether serotonin (5-HT) is involved in the nucleus raphe magnus (NRM) inhibition of cat multireceptive dorsal horn neurones, the NRM inhibition was tested with drugs (fluoxetine and pargyline) that should enhance 5-HT synaptic activity. 2. Fluoxetine and pargyline systemically administered, individually or concurrently, decreased rather than increased the inhibition. 3. The results do not support the hypothesis that NRM stimulation-produced inhibition is mediated by 5-HT release.