The evolutionary history of life on Earth has occurred under the omnipresent influence of a gravitational force. The exposure to the microgravity environment of space produces an array of biochemical and physiological changes in plants and animals. These changes extend from the cellular to the whole organism level. In order to manipulate gravity as an experimental variable and to separate the effects of weightlessness from the other variables in spaceflight, it is essential to provide a source of gravity in space. The scientific user community was consulted on the potential need and science requirements for a centrifuge to be designed for and flown on Space Station Freedom. Scientific purposes identified for the centrifuge were to: 1) provide a means of conducting fundamental studies in which gravity is a variable under experimental control, 2) provide a 1 -g control, 3) determine the threshold value of gravitation force for physiological response, 4) provide a source of 1 -g conditioned specimens that are also adapted to the spacecraft environment, and 5) determine the effects of intermittent hypergravity. Science requirements were generated which specified the largest possible diameter (approx. 2.5 meter), gravity levels ranging from 0.01 to 2 g, a nominal ramp-up rate of 0.01 g/sec, and life support for plants and animals. Based on the science requirements, a design concept for a centrifuge was developed at Ames Research Center. The use of artificial gravity in the microgravity environment of space will lead to an understanding of the importance of Earth gravity on living systems and the physiological adaptation to other levels of gravity.