The kinetic and pharmacological characteristics of the binding of [³H]5-HT (serotonin), [³H]8-OH-DPAT (8-OH-2- di-n-propylaminotetraline), [³H]LSD, [³H]ketanserin and [³H]mesulergine to membranes from frontal cortex, hippocampus and choroid plexus of pig brain were studied. The binding of these ligands to frontal cortex and hippocampus demonstrated the presence of 5-HT₁ and 5-HT₂ sites in both tissues, although hippocampus was richer in 5-HT₁ (subtype 5-HT_1A) sites. [³H]5-HT, [³H]mesulergine and [³H]LSD labeled the pig choroid plexus with high affinity. The pharmacological profiles of [³H]5-HT and [³H]mesulergine binding to this tissue were closely comparable. Ligands reported as selective for 5-HT_1A, 5-HT_1B or 5-HT₂ subtypes did not show high affinity for these binding sites. Therefore, these 5-HT binding sites in pig choroid plexus could be named 5-HT_1C. Other drugs with a high affinity for these sites were methysergide and mianserine. In pig frontal cortex, [³H]5-HT labeled the different subtypes of 5-HT₁ sites. In contrast, [³H]mesulergine bound in pig frontal cortex to a small population of sites with pharmacological properties similar to those of the choroid plexus 5-HT_1C sites. Possible physiological functions in which these sites might be involved are discussed.