When an earphone is held in the hand and placed tightly over the ear, a rumbling, low frequency noise can be heard. Since this sound is presumably caused by hand tremor, breathing, blood-circulation, etc., it is called physiological noise. It is possible, with a noise generator and low pass filters, to generate similar low frequency noises in an anechoic chamber, and to adjust the spectrum of the acoustical noise until a satisfactory qualitative match with the physiological noise is obtained. To make these matches, listeners used a circumaural cushion of the donut type which normally enclosed a volume of about 22 cc. Once a qualitative match was obtained, the listeners adjusted the intensity of the acoustical noise until it sounded just as loud as the physiological noise. These equal-loudness judgments indicated that the physiological noise was equivalent in loudness to an acoustical noise at a sensation level between 25 and 30 db. As the enclosed volume is increased, however, this level declines, and with volumes greater than about 1600 cc, physiological noise is seldom detected. This research was carried on under contract with the U. S. Navy, Office of Naval Research.