Corticosteroid hormones (aldosterone and glucocorticoids) and their receptors are now recognized as major modulators of cardiovascular pathophysiology, but their specific roles remain elusive. Glucocorticoid hormones (GCs), which are widely used to treat acute and chronic diseases, often have adverse cardiovascular effects such as heart failure, hypertension, atherosclerosis, or metabolic alterations. The direct effects of GC on the heart are difficult to evaluate, as changes in plasma GC concentrations have multiple consequences due to the ubiquitous expression of the glucocorticoid receptor (GR), resulting in secondary effects on cardiac function. We evaluated the effects of GR on the heart in a conditional mouse model in which the GR was overexpressed solely in cardiomyocytes. The transgenic mice displayed electrocardiogram (ECG) abnormalities: a long PQ interval, increased QRS and QTc duration as well as chronic atrio-ventricular block, without cardiac hypertrophy or fibrosis. The ECG alterations were reversible on GR expression shutoff. Isolated ventricular cardiomyocytes showed major ion channel remodeling, with decreases in I_Na, I_to, and I_Kslow activity and changes in cell calcium homeostasis (increase in C_al, in Ca²⁺ transients and in sarcoplasmic reticulum Ca²⁺ load). This phenotype differs from that observed in mice overexpressing the mineralocorticoid receptor in the heart, which displayed ventricular arrhythmia. Our mouse model highlights novel effects of GR activation in the heart indicating that GR has direct and specific cardiac effects in the mouse.--Sainte-Marie, Y., Cat, A. N. D., Perrier, R., Mangin, L., Soukaseum, C., Peuchmaur, M., Tronche, F., Farman, N., Escoubet, B., Benitah, J-P., Jaisser, F. Conditional glucocorticoid receptor expression in the heart induces atrio-ventricular block.