When applying a power line carrier system involving HVDC transmission, the HVDC system characteristics should be understood. Expected carrier noise levels on the DC line and on the adjacent, parallel or crossing AC lines must be known to ensure proper application of carrier. Primary information on noise measurements made on adjacent AC lines at Eel River, Stegall and Square Butte are presented. The need for noise filters on adjacent AC lines is discussed. A typical HVDC power line carrier system is also described and the computation of signal-to-noise ratio is given.