Background: Although magnesium deficiency induces left ventricular dysfunction, it is not known whether both systolic and diastolic functions are altered to the same extent. In this study, we investigated the effects of theophylline on left ventricular function in rats fed a normal diet or a magnesium-deficient diet for 1 month, and determined whether K201, a multi-channel blocker, modulated the effects of theophylline. Methods: Theophylline was infused at 5 mg/kg/min for 15 min in 6 control rats and 6 magnesium-deficient rats, and hemodynamic measurements were performed. In another 6 magnesium-deficient rats, K201 was infused at 0.1 mg/kg/min for 15 min simultaneously with theophylline. Results: Theophylline induced persistent increases in heart rate, peak positive first derivative of left ventricular pressure (+dP/dt), and a transient increase in left ventricular end-diastolic pressure (LVEDP), but did not affect left ventricular systolic pressure (LVSP) and peak negative first derivative of left ventricular pressure (-dP/dt) in the control rats. In contrast, in the magnesium-deficient rats, there was a persistent decrease in LVSP and a persistent increase in -dP/dt after theophylline infusion, although increases in heart rate, +dP/dt and LVEDP were similar to those in the control rats. When K201 was infused along with theophylline in the magnesium-deficient rats, both the decrease in LVSP and increase in -dP/dt were suppressed. Conclusions: Theophylline impaired left ventricular function in the magnesium-deficient rats, and this was improved by K201. K201 may provide new insights regarding future strategies for heart failure treatment.