Abstract: AIM:To investigate the effect of low-frequency pulsed magnetic fields (LF-PMFs) on proliferation and apoptosis of neonatal cardiomyocytes after hypoxia/reoxygenation (H/R) injury and the mechanism of NO/ONOO- balance involved. METHODS: Cultured neonatal cardiomyocytes were randomized into three groups: control group, H/R group, and LF-PMFs+H/R group. Viability and apoptosis of cardiomyocytes were assessed by MTT and TUNEL and levels of superoxide anion and NO/ONOO- were tested by Superoxide Assay Kit and Griess Assay, respectively. RESULTS: LF-PMFs promoted proliferation and inhibited apoptosis of neonatal cardiomyocytes following H/R treatment. Pre-/pro-treatment with LF-PMFs significantly downregulated the level of superoxide anion and ONOO- and upregulated NO expression of cardiomyocytes in vitro. LF-PMFs with 15 Hz, 4.5 mT yielded the most beneficial effect compared with magnetic fields at other levels. CONCLUSIONS: LF-PMFs exert protective effects on cardiomyocytes from H/R injury via balancing NO/ONOO-.