Junctin is a transmembrane protein of the cardia junctional sarcoplasmic reticulum (SR) that binds to the ryanodine receptor, calsequestrin, and triadin 1. This quaternary protein complex is thought to facilitate the SR Ca²⁺ release. To improve our understanding of the contribution of junctin to the regulation of SR function, we examined the age-dependent effects of junctin overexpression in atrium of 3-, 6-, and 18-week-old transgenic mice. The ratio of atrial weight and body weight was unchanged between junctin-overexpressing (JCN) and wild-type (WT) mice at all ages investigated (n = 6-8). The protein expression of triadin 1 was decreased starting in 3-week-old JCN atria (by 69%), whereas the expression of the ryanodine receptor was diminished in 6-week-old (by 48%) and 18-week-old (by 57%) JCN atria compared to age-matched WT atria. Force of contraction was decreased by 35% in 18-week-old JCN compared to age-matched WT left atrial muscle strips which was accompanied by a prolonged time of relaxation (48.1 ± 0.9 vs. 44.2 ± 0.8 ms, resp., n = 6-8), p<0.05). The spontaneous beating rate of isolated right atria was higher in 18-week-old JCN compared to age-matched WT mice (389 ± 10 vs. 357 ± 6 bpm, resp., n = 6-8, p<0.05). Heart rate was lower by 9% in telemetric ECG recordings in 18-week-old JCN mice during stress tests. Three-week-old JCN atria exhibited a higher potentiation of force of contraction at rest pauses of 30 sec (by 13%) and of 300 sec (by 35%), suggesting an increased SR Ca²⁺ content. This was consistent with the higher force of contraction in 3-week-old JCN (by 29%) compared to age-matched WT atria (by 10%) under the administration of caffeine. We conclude that in 3-week-old atria junctin overexpression was associated with a reduced expression of triadin 1 resulting in a higher SR Ca²⁺ load without changes in contractility or heart rate. In 6-week-old JCN atria, the compensatory down-regulation of the ryanodine receptor may offset the effects of junctin overexpression. Finally, the progressive decrease in the ryanodine receptor density may contribute to the decreased atrial contractility and a lower heart rate during stress in 18-weekold JCN mice.