Abstract: AIM To study the changes of ultramicrostructure in atrial myocytes and the expressions of mitogen-activated protein kinases (MAPKs) after rapid pacing. METHODS Rat primary atrial myocytes were cultured and a rapidly paced cell model was established. The atrial myocytes were divided into rapid pacing group and control group. Western blot was employed to detect the changes of protein expressions of extra-cellular signal regulation kinase (ERK), p38MAPK and their phosphorylation level, respectively. The ultramicrostructure in atrial myocytes was observed under transmission electromicroscope at 24 hours after rapid pacing. RESULTS Expressions of ERK, p-ERK and p-p38MAPK were all upregulated at 24 hours after rapid pacing, compared to those before pacing （P<0.01）, however, no significant change was observed in the expression of p38MAPK. The ultramicrostructure in atrial myocytes displayed glycogen aggregation, karyopycnosis, sarcoplasmic reticulum distention and chondriosome swelling at 24 hours after rapid pacing. CONCLUSION Obvious change of ultramicrostructure in primarily atrial myocytes takes place in the early phase of rapid pacing and at the same time ERK and p38MAPK are activated. It may be an important mechanism of structure remodeling of atrial myocytes in the early of atrial fibrillation.