Interference voltages may be picked up during Electrocardiograph (ECG) recording. The possible sources of these interference voltages in the low voltage biomedical instrumentation circuit have been discussed in the paper. A case study has been made where appreciable signal to noise ratio has resulted in deterioration in ECG recording. The preventive methods have been reviewed to reduce the picked up interference voltages which reappear in the form of noise voltages. The case study also points out some interesting applications of the ECG records.