Background The Mesenchymal stem cells (MSCs) can provide paracrine growth factor support for other cells in injured myocardium. We hypothesized that the anti-arrhythmic effect of MSCs might depend on the environment through different paracrine action. Methods Sham (n=23), paracrine media (PM) secreted from MSC under hypoxia (hypoxic PM group, n=12), PM secreted under normoxia (normoxic PM group, n=14) were administrated at border zone in rats with MI. At 7-10 days, we performed LV catheterization and optical mapping. We identified paracrine effect of MSCs by histological analysis, immunostaining, measurement of cytosolic free Ca2+, RT-PCR, and proteomics analysis. Results- Survival curves revealed a marked reduction of sudden deaths in hypoxic PM, compared to normoxic PM (8.3% in hypoxic PM vs. 35.7% in normoxic PM, p=0.1) and sham-operated group (8.3% in hypoxic PM vs. 52.2% in sham, p=0.016). In optical mapping, VT induction protocol induced VT in 77.8 % of sham-operated (n=9), 14.3 % of hypoxic PM (n=7), and 50 % of normoxic PM group (n = 6). Conduction velocity (CV) was markedly restored in hypoxic PM treated lesion compared to normoxic PM treated lesion (0.8짹0.1m/s vs. 0.37짹0.09 m/s, p<0.05) and sham-injected lesion (0.1짹0.1 m/s). The focal activity emanating from border was suppressed in hypoxic PM compared to normoxic PM (0% vs. 33.3 %, p<0.05) or sham group (55.6%). A decrease of fibrosis, restoration of connexin 43, alleviation of Ca2+ overload, and an increase of Ca2+ related protein expression underlay this electrophysiological alteration. It was identified to be mediated by substances such as bFGF, IGF-1, HGF and EF hand domain containing. To assess the cellular effect of MSCs, we compared MSCs -injected rats (m=24) with normoxic PM-treated rats (n=15) after 1hr injury and treatment. CV were similar in MSC engrafted and normoxic PM treated lesions (0.42짹0.1m/s in MSCs vs. 0.37짹0.09 m/s in normoxic PM, p>0.05), but APD80 increased in MSCs-engrafted lesion (98짹12 ms vs. 78짹21ms, p<0.05). Survival showed no significant difference (75% in MSCs vs. 67% in normoxic PM, p>0.05). Conclusions-PM secreted from MSCs showed different preventive effect in sudden death and arrhythmogenicity according to hypoxic or normoxic environment, suggesting that antiarrhythmic effect of MSCs may depend on the surrounding environment.