AIMS: Pronounced sex-differences are known in the incidence of atrial fibrillation (AF). In this study, we aimed to investigate the atrial electrophysiological properties that may underlie sex-differences in AF incidence in the younger population, focusing on IK1, a cardiac ion current important for action potential (AP) stability and triggered activity. METHODS AND RESULTS: We assessed sex-differences in P-wave morphology in 12-lead ECG in healthy young New Zealand White rabbits. Males presented longer PWD and larger P-wave area compared to females. Patch-clamp experiments were performed in isolated rabbit atrial cardiomyocytes (CMs). Male atrial CMs presented higher delayed after depolarizations (DAD) incidence, amplitude, and area under the curve (AUC) than females, potentially facilitating the presence of atrial triggered activity in males. Male atrial CMs showed a less hyperpolarized resting membrane potential (RMP), a 50% smaller IK1, and a 26% reduction in Kir2.1 protein expression, a pore forming subunit of IK1, than females. Dihydrotestosterone (DHT) effects were investigated acutely and semi-chronically ex vivo. Experiments showed that the sex-difference in IK1 could be mimicked by DHT. In female atrial CMs, acute and semi-chronic (24†h) DHT administration reduced IK1. In the presence of a PKC-inhibitor, DHT-mediated IK1 reduction was not observed in atrial female CMs, suggesting it to be PKC-mediated. Chronic DHT-effects were investigated in vivo in female rabbits after hormone-releasing pellet implantation. After 2 weeks, animals showed a significantly prolonged and larger P-wave, a smaller atrial IK1 and a trend towards an increased DAD amplitude and AUC. CONCLUSION: Sex impacts on atrial electrophysiology, leading to sex-differences in P-wave morphology, triggered activity, RMP, and IK1. These sex-differences can be mimicked by sex hormone-treatment, suggesting that sex hormones-particularly DHT-play a pivotal role in mediating sex-differences in atrial electrophysiology. Such sex-differences might impact on the propensity to develop AF, particularly in the younger population.