The present study was planned to investigate the effects of two different concentrations (10 and 40 mg/kg) of monensin, carboxylic ionophore antibiotic, on the ECG and the serum Na+ and K+ concentrations of rabbits. Fifteen male rabbits, about 2 kg and 1 year old, were used. ECGs were recorded by a direct writing electrocardiograph. Blood samples for measuring serum Na+ and the K+ concentrations were collected and the leads I, ii, II, aVR, aVL and aVF were recorded before and 6 hours after monensin was given. The durations and amplitudes of waves on the trace were measured in lead II and electrical axis also measured in leads 1 and III. We found that the effects of monensin on ECG and the serum Na+ and the K+ concentrations were dose dependent. In low doses (10 mg/kg) it produced a sinusoidal tachycardia by decreasing the time interval between two impulse production by sinoatrial node without changing the serum ion balance. However, in high doses (40 mg/kg) it produced a sinusoidal tachycardia with increasing the serum K+ and decreasing the serum Na+ concentrations.