A catheter assembly designed for long term or short term implantation in an animal body comprising a flexible tube of a biocompatible polymeric material in which plural electrical conductors are helically wound at a predetermined pitch with the conductors being laterally offset from one another and totally buried between the walls of the tube whereby many conductive signal paths can be established through the catheter without increasing its overall diameter. The inclusion of the helically wound conductors in the walls of the tube also allows the torque transfer, flexibility and structural properties to be tailored to fit a variety of applications. Such a catheter may be used as a cardiac pacer lead assembly or as an instrument for carrying out various diagnostic catheterization procedures.