Sixty-four fertilized chicken eggs, half at developmental Day 2 and half at Day 9, were exposed to micro-gravity for 5 days aboard the shuttle. Postflight examination showed that none of the Day 2 flight embryos had survived, whereas the Day 9 flight group and both groups of synchronous ground control embryos appeared viable. One-half of the Day 9 flight and ground control embryos were dissected and the temporal bones preserved in acetone for morphological examination. The other half was allowed to hatch to examine vestibularly related behavioral changes. Morphology of the lagenar otoconia was evaluated by scanning electron microscopy. Behavioral changes were accessed by a battery of reflex tests and recordings of spontaneous and vestibularly driven head movements. The results from both the morphological and behavioral studies showed no consistent difference between the flight and the control animals. Several hypotheses may account for this negative result. Because all the Day 2 embryos failed to survive, the remaining Day 9 chicks may have passed the critical developmental period of the chick's vestibular system. Also, the reexposure of the developing chick embryo to earth's 1-g environment may have masked any adverse behavioral effects that exposure to Microgravity may have caused.