A cylindrical barium titanate element has been placed at the distal end of a specially designed catheter and used in studies of heart sounds of dogs and humans. The heart sounds have been recorded with the sensitive catheter located directly in the heart and in the pulmonary artery. The location of the catheter at each mapping station has been monitored with x‐ray photographs; simultaneous electrocardiogram and wide band sounds were recorded on a photographic galvanometer. The sounds were recorded on tape with several filter characteristics, and acoustic spectrographs were made. These studies indicate that much may be learned from an analysis of the spectra and time characteristics of sounds picked up directly within the circulatory system. [Work supported in part by National Institutes of Health H‐1425(c).]