Smith has implicated social factors as interfering with acceleration effects. His subjects were typically accelerated four or six to a cage. When groups were mixed midway through the experiment, they exhibited a higher mortality rate and incidence of acceleration symptoms than did groups whose constituency remained unchanged. At the U.S. Naval School of Aerospace Medicine, numerous studies have been conducted on the effects of slow rotation on the behavior and physiology of humans and animals ([ref.100]). Rotation initially produces decrements in performance, but adaptation to a rotating environment ensues quite rapidly (refs. [ref.101]-[ref.103]). Perceptual distortion, nystagmus, nausea, and other signs of discomfort are common responses to slow rotation. These symptoms are generally reduced with continued exposure (adaptation). Interestingly, however, adaptation is delayed when the subjects are exposed to a fixed reference outside their rotating environment. At NASA-Ames, rodents have been used in experiments by Weissman and Seldeen to delimit the stimulus effects of rotation. In these experiments the subjects must discriminate between different speeds of rotation in order to obtain food reinforcement. The results thus far provide evidence that these animals are capable of discriminating between the different speeds at which they are being rotated. The range of speeds studied was 0-25 rpm, with tests of discrimination being made at intervals of less than 5 rpm. Experiments such as these will lead to the development of techniques for measuring rotational sensitivity in many species, including man. The optimum configuration of manned spacecraft will depend, in part, upon biomedical considerations. A voluminous literature now exists on the possible hazards to man of prolonged exposure to zero-g conditions. Should prolonged weightlessness prove to be a serious detriment to health, consideration must be given to design concepts which provide artificial gravity. No data exist on the minimum gravity requirements necessary to sustain basic biological functions for extended periods. A limit of 0.2 g has been given as the lower level at which man can walk unaided ([ref.104]). It has also been recommended that angular velocity be maintained at the lowest possible level in order to minimize the occurrence of vestibular disturbances. These recommendations are based on human-factor requirements, rather than upon biological considerations, which may significantly modify these values. In recent studies, a technique has been devised which promises to provide reliable criteria for biological acceptability, since it is based on fundamental biological and behavioral principles.