The purpose of the present study was to determine whether the 5-ht6 receptor is functionally expressed in the rat brain by blocking its translation from mRNA with treatments of phosphorothioate antisense oligonucleotides. Rats were treated with either saline, antisense (AO) or scrambled oligonucleotides (SO) for 4 days. Treatment with AO reduced the number of [3H]LSD binding sites in the frontal lobes by 30% but had no significant effect on the number of 5-HT1A and 5-HT2A receptor binding sites in the cortex of the rats. A behavioural syndrome of yawning, stretching and chewing, however, was observed in AO treated rats but not in any of the other treatment groups. This AO-specific behaviour had returned to normal 5 days after cessation of the oligodeoxynucleotide treatment. These data suggest that the 5-ht6 receptor has a physiological function in the rat brain where it appears to be under the tonic control of endogenous 5-HT.