The heat introduced into a cryogenic system by current-carrying leads has been investigated experimentally and theoretically. The experimental work consisted of measuring the heat conducted by various cables and wires into a helium Dewar. The experimental results show that the use of long coaxial cables to the helium bath minimizes boil-off and also that there are optimum wire sizes for the current ranges 0-0.5 amp (#38 AWG) and 0.5-1.0 amp (#34 AWG). The theoretical analysis of the heat input takes into account the varying electrical and thermal conductivities of the leads and also allows for heat exchange with the effluent helium vapor. The results of this analysis indicate the importance of including gas cooling in predicting the boil-off rate. © 1962 The American Institute of Physics.