Objective.To investigate the protective effects of melatonin（MT ）on cardiomyocytes against the ox-idative injury induced by H 2 O 2 .Methods.Ventricular myocytes were isolated from neonatal rats and cultured for3～5days.Cells were divided into4groups:control group

H 2 O 2 （100μmol/L H 2 O 2 ）group

MT group（100μmol/L MT ）and MT preconditioning group（100μmol/L MT+100μmol/L H 2 O 2 ）.The fluorescent probe

DCFH?DA

was used to detect intracellular levels of reactive oxygen species（ROS）and another fluorescent probe

Fluo-3?AM

was used to detect[Ca 2+ ] i by using a laser scanning confocal microscopy（LSCM）.The malondi-aldehyde（MDA）content in cardiomyocytes was determined by measuring thiobarbituric acid?reactive sub-stances to monitor lipid peroxidation.The activity of cytoplasmic enzyme lactate dehydrogenase（LDH）that was released into the culture media was assayed to indicate alternation in the integrity of the cellular membrane.Trypan blue exclusion was used to detect the cell viability.Results.Compared with the control group

intracellular ROS

[Ca 2+ ] i

MDA content

LDH leakage and cell death were significantly elevated when cells were treated by100μmol/L H 2 O 2 for60minutes（P<0.01）.However

those changes were significantly attenuated in MT preconditioning group.Conclusion.MT has very good antioxidant effect and can protect the cardiomyocytes against H 2 O 2 -in-duced injury.