Dr. Muriel Ross showed on SLS-1 and SLS-2 that the maculas from the inner ear of rats flown in space have considerably greater numbers of synapses than their Earth controls. This important evidence for neural plasticity in a gravity sensing area is being investigated further. Dr. Ross will study the inner ear of rats on Neurolab and Dr. Gay Holstein will study how the neural circuits in the vestibular system have changed. Dr. Chuck Fuller will measure circadian rhythms in heart rate and temperature in rats, as well as delineate the precise location in the brain where the rhythms are regulated. Dr. Ottavio Pompeiano will study both vestibular and circadian timing areas of the brain. The unique environment of space will be used to reveal how the rat determines its location. Dr. Bruce McNaughton and his colleagues at the University of Arizona have devised a unique track that rats will walk upon in zero-G to determine how the nervous system will resolve the conflict between input to the eyes and the ears. The encoding of location in the hippocampus of chronically instrumented rats will be measured.