Low‐frequency noise of physiological origin has been measured in a cavity enclosing the ear. The measurements were performed with a good condenser microphone resiliently mounted in a special enclosure of effective volume approximately 60 cm3sealed to the head with a circumaural liquid‐filled cushion. The over‐all noise level in the enclosure and levels in certain bands rise and fall by a few decibels at the heart‐pulse frequency, indicating that the noise is generated by the circulatory system. The level per13octave band averaged over six subjects is approximately 70 db re 0.0002 dyne/cm2at 16 cps falling steadily to 34 db at 125 cps and 12 db at 250 cps. With the 12‐cm3effective volume of a TDH39 and MX41AR cushion, one would anticipate physiological noise levels 14 db higher than these or 48 db at 125 cps and 26 db at 250 cps. The levels per critical band might be somewhat higher yet. Since the tentative ISO hearing threshold levels at 125 and 250 cps are 49 and 27 db, respectively, one is led to suspect that these levels may have been raised by the masking effect of physiological noise.