We analyzed various organs in the same rats to study effects of gravitational condition on organ development of the neonatal rat in this study. Eight-day old and 14-day old Sprague-Dawley rats were flown for 16 days on the Space Shuttle Columbia (April 17-May 3, 1998). The organs were weighed and the ratio of the organ weight to the body weight (organ weight ratio; OBR) was calculated. Tissues were analyzed using anatomical, immunohistochemical and molecular biological technique. Six animals of the 8-day old group were reared on the ground for 30 more days after landing. The differences between flight and control rats in 8-day group were drastic. The lung, heart, kidney and adrenal glands in flight rats were significantly larger than that of control rats in OBR comparison. However, only the lung and kidney were still larger after 30 more days on ground. The kidney in flight rats performed pelvis expansion with down-regulation of aquaporin-2 expression confirmed by immunohistochemistry. The thymus, spleen, mesentery and pancreas were smaller in OBR. But the thymus in flight rats was heavier after 30 more days. The organs in flight rats which had no differences in OBR showed normal characteristics in histological analysis. We also found that the number of unmyelinated fibers of the aortic nerve in flight rats of 8-day group was smaller than that in control rats. In flight rats of the 14-day group, only the kidney was heavier and the ovary was lighter as compared to the controls. These results implied the second week of life was important for development during spaceflight. And the sensitivity and the critical period on neonatal development under microgravity might differ in each organ.