I T HAS BEEN known for many years that stimulation of the non-acoustic labyrinth may produce stress of varying degrees in humans and animals and that head movements during bodily rotation are peculiarly stressful. Students of semicircular canal function have found, therefore, that it is necessary to control carefully head movements in all studies involving vestibular function. More specifically, it has been shown 5 that controlled head movements induced during rotation at constant speed cause severe "motion sickness" in healthy individuals. Studies of stress resulting from the stimulation of semicircular canals by head movements have important implications with regard to the psychophysiological mechanisms involved. They also have practical implications in connection with proposed orbiting vehicles because it has been suggested I,~,7,9 that these might be rotated at a constant angular velocity in order to produce an artificial gravitational force. In the part of the present investigation reported earlier 4 the authors described a method whereby the consequences of rotation for fortyeight hours in a slowly rotating room could be determined. The effective stimulus was the