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Objective The chemokine stromal cell-derived factor-1 (SDF- 1) has been implicated in homing of bone marrow cells to sites of injury. We investigated the time course of myocardial SDF-1 expression and effects of intravenously administered bone marrow mesenchymal stem cells (MSC) in rats with myocardial infarction (MI). Methods SDF-1 expression was measured by RT-PCR and Western blot in sham operated or infarcted hearts at 1/2, 1, 2,4, 8 and 16 days post operation. MSCs from donor rats were labeled with BrdU. A total of 5 × 106 cells in 2.5 mL of PBS or equal volume PBS alone were injected through the tail vein at above mentioned time points. The number of the labeled MSCs in the infarcted hearts was counted 3 days post injection. Cardiac function and vessel numbers were assessed 28 days post injection. Results Myocardial SDF-1 expression increased and peaked at the first day and decreased thereafter post MI and remained unchanged in sham operated hearts. The MSCs enrichment and angiogenesis in the host hearts were more abundant in the 1 day transplantation group than in the other groups (P < 0.01). Cardiac function was only improved in rats received intravenous MSCs injection within 4 days post MI and not affected by PBS injection. Conclusions Myocardial SDF-1 expression was increased only in the early phase post MI. MSCs intravenous infused at the early phase of MI were recruited to injured heart, enhanced angiogenesis and improved cardiac function. [ABSTRACT FROM AUTHOR]