Introduction: ATF6 is an endoplasmic reticulum (ER) transmembrane protein that is activated upon accumulation of misfolded proteins in the ER during ER stress. Upon ER stress, the cleaved N-terminal fragment of ATF6 translocates to the nucleus and acts as a transcription factor, regulating a widespan of adaptive genes to restore proteostasis in the ER, thus promoting cell survival. There are two isoforms of ATF6, ATF6α and ATF6β, which have high sequence homology. While activation of ATF6α is protective in the heart under different pathologies, the precise role of ATF6β in cardiac myocytes is not yet known. Hypothesis: ATF6β is an adaptive transcription factor in cardiac myocytes. Methods: The role of ATF6β was studied using cultured primary neonatal rat ventricular myocytes (NRVM) as a model upon which ATF6β loss- or gain-of-function approaches, such as siRNA-mediated knockdown or ectopic expression, respectively, were implemented. The effects of these maneuvers were examined in NRVM cultured for 1-4 days ± treatment with ER-stress inducer, tunicamycin (TM). MTT assays were used to determine the effects of these maneuvers on cardiac myocyte viability. Quantitative real-time PCR was used to assess the expression of members of an ER stress-inducible gene panel. To determine whether the effects of ATF6β on viability were due to its ability to induce transcription or to interact with other transcription factors, DNA constructs encoding various forms of mutated ATF6β were generated, then tested for expression in HeLa cells. Results: In NRVMs, ATF6β loss- and gain-of-function significantly decreased and maintained cardiac myocyte viability basally and upon treatment with TM, respectively. Gene expression analyses in both ATF6β loss- and gain-of-function models suggested that ATF6β exerts its protection by regulating some adaptive genes and by repressing at least one maladaptive gene. In HeLa cells, the first 115 amino acids in the N-terminal domain and the DNA-binding domain of ATF6β are both critical to its transcriptional activity. Conclusion: The effect of ATF6β loss- and gain-of-function on NRVM viability and gene expression demonstrate that ATF6β has an adaptive role in cardiac myocyte viability and suggest a repressive role of ATF6β on maladaptive gene expression to achieve its protective effect.