Plasma digoxin level is at best an indirect measure of pharmacological response to digoxin in patients being treated for congestive heart failure. Systolic time interval (STI) measurement reflecting left ventricular function at the physiological site of action of digoxin, is both more direct and non-invasive. With a portable instrument to measure systolic time intervals, the measurement can also be convenient for hospital staff. A portable electrocardiogram (ECG) machine was modified to mimic the capabilities of a large, multichannel model. Upon satisfactory evaluation, it was employed in the collection of systolic time interval data from five hospitalized patients undergoing digoxin treatment. An attempt was made to show a relationship between STI indices and digoxin plasma concentrations. Additionally, a statistical comparison was made of change in STI (delta STI) and plasma digoxin concentration both before and after drug administrations. The change in pre-ejection period (delta PEP) values show a significant difference over the changes in total electromechanical systole (delta QS2) and the changes in left ventricular ejection time (delta LVET). In three congestive heart failure patients, the time course of the change in plasma concentration showed good correspondence with delta PEP.