Abstract: AIM: To observe the effect of cyclosporine A (CsA) on the morphology and function of myocardial cells in vitro and to study the mechanism underlying CsA-induced injury in myocardial cells. METHODS: Myocardial cells of neonatal rats were cultured for 48 h with 100 ml/L DMEM and were randomly divided into four groups: normal control group and CsA treated groups (with 103 μg/L, 104 μg/L and 105 μg/L CsA). Twenty-four h after the treatment, cellular morphological changes were observed by inverted phase contrast microscope and electron microscope, respectively. MTT was adopted to measure the cell numbers. Lactate dehydrogenase (LDH) levels and malondialdehyde (MDA) levels in the supernatants of the cultured myocardial cells in each group were detected by colorimetric method and TBA 24 h and 48 h after treatment. RESULTS: The numbers and beats of myocardial cells in CsA-treated groups signigficantly decreased compared with those in control group. Vacuolizations were observed in the endochylema of myocardial cells in CsA-treated groups. Ultrastructure of cardiomyocytes was seriously damaged and mitochondrial swelling denaturation was found in CsA- treated groups. The cell numbers and activities in CsA-treated groups were all lower than those in control group (P<0.05, P<0.01). LDH and MDA levels in CsA-treated groups were higher than those in control group at 24 h and 48 h after treatment in a significant concentration- and time-dependent manner (P<0.05). CONCLUSION: CsA induces myocardial cell injury in a dose- and time-dependent manner, the mechanism of which may be associated with the damage of lipid peroxidation.