Serotonin (5-hydroxytryptamine, 5HT) is a monoamine neurotransmitter that is also abundant outside the central nervous system where it circulates in blood platelets and mediates a broad variety of neuronal and non-neuronal biological functions. 5HT is produced in the mammalian reproductive tract, oocyte and preimplantation embryo where it may participate in the regulation of fertilization and early embryonic development. Preimplantation mouse embryos express several G protein coupled 5HT receptors, as well as the 5HT reuptake transporter, and are potentially capable of serotonergic signaling. 5HT is generated from tryptophan by tryptophan 5-hydroxylase (EC 1.14.16.4), an enzyme encoded in mice by two genes, Tph1 and Tph2, which are restricted in their expression to tissues either outside or within the central nervous system, respectively. Interestingly, Tph2, but not Tph1, is expressed in oocytes and preimplantation embryos. Transgenic mice homozygous for the deletion of Tph2 had a significant (14-fold) increase in pregnancy failure that was also evident in periimplantation development between embryonic day 4.5 (E4.5) and E7.5. However, morula-stage embryo (E2.5) production was not significantly affected by a lack of embryonic TPH2. TPH2 and 5HT were detected by immunofluorescence in wild type embryos from the 1-cell to morula stage, but decreased abruptly at the blastocyst stage. Developmental rates in vitro were similar for wild type and Tph2 null morulae to the hatched blastocyst stage, but trophoblast differentiation into migratory cells was delayed by about 24 hours (p<0.05) in knockout embryos cultured to form blastocyst outgrowths. Tph2 null mothers are able to produce 5HT in the reproductive tract using TPH1, but this was insufficient to support normal development of Tph2 null embryos. However, pregnancy failure rates were intermediate in Tph2 null mothers crossed with heterozygous males or in heterozygous mothers crossed with Tph2 null males, suggesting at least a partial rescue in vivo by 5HT-producing sibling embryos. We conclude that in the absence of endogenous 5HT synthesis, fertilization and preimplantation embryo production is normal, but implantation and pregnancy success rates are both reduced. These findings suggest that autocrine or local paracrine serotonergic signaling during preimplantation development, prior to blastocyst formation, ensures on time differentiation of the trophoblast for implantation. Activation of embryonic 5HT receptors could mediate interactions among blastomeres or induce gene expression that is required for trophoblast functions that take place during or after blastocyst implantation to maintain a viable fetus. The Tph2 null mouse is a unique model of implantation failure arising from trophoblast dysfunction that might also provide insight into placental insufficiency and associated perinatal disorders. Supported by the Intramural Research Program of the NICHD, NIH, and the Department of Veterans Affairs.