A new system for recording intracardiac and intravascular sound during routine catheterization is described.This system is based on the application of a flat, sonar-type, barium titanate microphone at the end of the catheter, so that the vibrations of the blood within the chambers of the heart and vessels are transmitted along the fluid in the catheter, and transduced by the microphone (fluid column transmitted phonocardiogram-FCT phonocardiogram).Comparative studies between the response of the FCT system and other systems for intracardiac phonocardiography are made in normal dogs and in dogs with experimental aortic lesions. It is demonstrated that the FCT system gives a good response for vibrations between 25 and 400 cycles per second.Representative phonocardiographic tracings in patients with congenital or acquired heart disease during routine cardiac catheterization are presented.