In the adult, in various conditions (e.g. myocardial infarction, heart failure and pulmonary hypertension) there is sinoatrial node (SAN) dysfunction as well as changes in the expression of Tbx3. Tbx3 is known to play an important role in the embryonic development of the SAN, but the role in the adult is not known. Here, we explore the effects of Tbx3 upregulation in the adult SAN. Tbx3 was upregulated in the SAN by crossing CAG-CAT-Tbx3 transgenic mice with heterozygous HCN4-kit transgenic mice: this generates double-transgenic mice conditionally expressing Tbx3 in the SAN. Tbx3 upregulation was induced by intraperitoneal injection of tamoxifen (Sigma T5648; 40 mg/kg) for 3 days into adult males. The mice were kept for 3 weeks after the last injection. Animal procedures were undertaken in strict accordance with the United Kingdom Animals (Scientific Procedures) Act 1986. Data are presented as means±SEM & statistical differences assessed by Student’s t test, one – way ANOVA or two – way ANOVA as appropriate. Differences were considered significant if P<0.05. Quantitative PCR revealed an upregulation of Tbx3 mRNA in the SAN as expected. The ECG measured in the conscious and anaesthetised animal showed significant sinus tachycardia (increase in heart rate of 85±29 and 120±43 beats/min, n=10). In isolated SAN preparations, beating rate was significantly shorter in Tbx3 upregulated mice by 7.5%. SAN pacemaking is determined by the membrane and Ca2+ clocks. The membrane clock may not be responsible for the sinus tachycardia: transcripts of three key ion channels (all involved in the membrane clock) were investigated – HCN1 and Nav1.5 were unchanged, whereas HCN4 was significantly downregulated by 45%. However, the Ca2+ clock may be responsible: transcripts of three key Ca2+ clock components were investigated – RYR2 and NCX1 were unchanged, whereas SERCA2 was significantly upregulated by 152%. We investigated transcripts for a number of transcription factors known to play an important role in the SAN. Mef2c and Tbx18 were unchanged, whereas Rest1 (represses HCN4 expression) was significantly upregulated by 26%, Pitx2 was significantly upregulated by 87% and Nfat4 was significantly upregulated by 730%. A potential Tbx3 binding site on the promoter region of Nfat4 was identified by in silico analysis. Nfat4 is known to upregulate SERCA2 (Prasad and Inesi, 2011). In conclusion, Tbx3 is able to control SAN pacemaking in the adult possibly by controlling SERCA2 expression via Nfat4.