The need for space centrifuges to simulate gravitational loading for biological space experiments has been recognized since the inception of the space program. The enhanced scientific opportunities provided by the long-term space exposure capability of a space station increases the value and need for such centrifuges. A space station biological facility should include the capability to allow both acute and extended chronic exposure of test subjects and biological materials to altered G-loading. This should include both chronic 1 G acceleration to provide a control environment for test subjects and chronic acceleration in the range of 0-1 G to determine gravitational thresholds of the different systems being examined. Major scientific problems and questions concerning the effects of gravity and especially weightlessness on plant and animal physiology and development will be discussed in terms of experimental approaches that demand a space station centrifuge for their resolution.