Two polymorphic dinucleotide repeats separated by a short spacer are localized in the promoter region of the serotonin receptor 2C gene ( HTR2C). One of the repeats was found to be evolutionary conserved between humans and rhesus monkeys. Although promoter-associated microsatellites have previously been shown to regulate expression of different genes, we did not find any significant influence of distinct HTR2C promoter microsatellite alleles on transcriptional efficiency as measured by luciferase activity and receptor availability as assayed by [(3)H]-mesulergine binding. Furthermore, no association of specific alleles with bipolar disorder was found. These results indicate that the HTR2C promoter polymorphism does not contribute significantly to the etiopathogenesis of bipolar disorder in females.