Serotonin (5-HT) is one of the well known monoamine neurotransmitters, mitogen, and hormones, which mediate a wide variety of physiological effect, including peripheral and central actions. The large variety of 5-HT functions is paralleled by the pharmacological complexity of 5-HT receptors which can be classified into different families depending on their signalling pathways. A study by 5-HT2B antagonists showed 5-HT and 5-HT2B receptor function in development of tooth germ. To elucidate the role of them, I have examined the localization of 5-HT immunohistochemically and the expression of 5-HT2B subtype receptor by in situ hybridization in rat tooth germ. At the bud stage both 5-HT and 5-HT2B mRNA were not detected in incisors and molars. At the cap stage both 5-HT and 5-HT2B mRNA were detected in the enamel organ and dental papilla. 5-HT were intensely localized at the inner enamel epithelium. Otherwise 5-HT2B mRNA were intensely expressed in the mesenchymal cells of the dental papilla. At 1 day postnatal presence of 5-HT and 5-HT2B mRNA were also shown in the tooth germ of incisor and molar. However their intensity were decreased gradually in the process of differentiation of ameloblasts and odontoblasts. These results indicate that 5-HT and 5-HT2B play a role in tooth development.