The radio-telemetric technology makes it possible to continuously monitor arterial pressure, heart rate and physical activity as well as their circadian rhythms and their response to therapeutics in unrestrained animals for a longer period of time. This is essential for in vivo cardiovascular research. The primary purpose of the present study was to use telemetry to characterize those physiological parameters in a rat model of heart failure post myocardial infarction. Moreover, some of the neurohormonal alterations and the hemodynamic responses to L-arginine, the substrate for nitric oxide synthase (NOS), and L-NAME that inhibits NOS activity were also evaluated. Rats with moderate and large myocardial infarction are considered to have chronic heart failure (CHF), which displayed decreased systolic and consequently decreased mean blood pressure, blunted diurnal dips in systolic, diastolic and mean blood pressure, narrowed pulse pressure and relatively reduced strength of physical activity. These telemetric alterations were accompanied by increased left ventricular end diastolic pressure, right and left ventricular hypertrophy and elevated plasma norepinephrine and tumor necrosis factor. In CHF rats, L-arginine widened pulse pressure and L-NAME caused attenuated pressor response, suggesting the presence of the general NO deficiency in this model. Thus, telemetry provides some new and unique information about CHF in rats, which is important in exploring the underlying mechanism of heart failure, but cannot be obtained by the conventional approach.