Previous studies have shown that the changes seen in the bones of growing rats exposed to microgravity are due in part to changes that occur in the growth plate during spaceflight. In this study, growth plates of rats flown aboard Cosmos 1887 (12.5‐day flight plus 53.5‐h recovery at 1g) were analyzed using light and electron microscopy and computerized planimetry. The proliferative zone of flight animals was found to be significantly (P≤ 0.01) larger than that of controls, while the reserve and hypertrophic/calcification zones were significantly reduced. Flight animals also had more cells per column in the proliferative zone than did controls and less in the hypertrophic/calcification region. The total number of cells, however, was significantly greater in flight animals. No difference was found in perimeter or in shape factor, but area was significantly less in flight animals. Electron microscopy showed that collagen fibrils in flight animals were wider than in controls. Since the time required for a cell to cycle through the growth plate is 2‐3 days at 1g, the results reported here represent both the effects of exposure to microgravity and the initial stages of recovery from that exposure.—Duke, P. J.; Durnova, G.;andMontufar‐Solis, D. Histomorphometric and electron microscopic analyses of tibial epiphyseal plates from Cosmos 1887 rats.FASEB J.4: 41‐46; 1990.