BACKGROUND: Diabetes mellitus induces life-threatening cardiovascular complications such as cardiac autonomic neuropathy and ventricular dysfunction and is associated with hypomagnesemia. In this study, we investigated the short-term effects of magnesium (Mg(2+)) treatment on streptozotocin (STZ)-induced diabetic cardiac complications. METHODS: Adult Wistar rats were treated once with STZ (50 mg/kg, intraperitoneally [ip]) or vehicle (citrate) and then daily for 7 days with MgSO(4) (270 mg/kg, ip) or saline. On the eighth day, in vivo tail-pulse plethysmography was recorded for heart rate variability (HRV) analysis, and ex vivo Langendorff-based left ventricular (LV) pressure-volume parameters were measured using an intraventricular balloon. Measurements of plasma lipid and Mg(2+) levels as well as blood glucose and cardiac tissue Mg(2+) levels were also performed. RESULTS: Treatment with Mg(2+) prevented diabetes-induced alterations in the standard deviation of the averages of normal-to-normal (NN) intervals (SDANN), root mean square differences of successive NN intervals (RMSSD), heart rate, and low-frequency (LF) power-high-frequency (HF) power ratio. In addition, Mg(2+) restored orthostatic stress-induced changes in SDANN, RMSSD, and LF-HF ratio in diabetic rats. In isolated hearts, Mg(2+) reversed the diabetes-induced decrease in LV end-diastolic elastance and the right shift of end-diastolic equilibrium volume intercept, without altering LV-developed pressure or end-systolic elastance. However, Mg(2+) did not prevent the elevation in blood glucose, total cholesterol, and triglycerides or the decrease in high-density lipoprotein cholesterol in diabetes. Plasma- or cardiac tissue Mg(2+) was not different among the treatment groups. CONCLUSION: These results suggest that Mg(2+) treatment may attenuate diabetes-induced reduction in HRV and improve LV diastolic distensibility, without preventing hyperglycemia and dyslipidemia. Thus, Mg(2+) may have a modulatory role in the early stages of diabetic cardiovascular complications.