The influence of 12.5 days of spaceflight and a 55 h stressful recovery period (at 1g) on fibroblastlike osteoblast precursor cells was assessed in the periodontal ligament (PDL) of rats that were 91 days old at launch. Nuclear morphometry was used as a marker for precursor cell differentiation in 3 μm sections cut in the midsagittal plane from the maxillary first molar. According to nuclear volume, cells were classified as pre‐osteoblasts (C + D cells, ≥120 μm3) and less differentiated progenitor cells (A+A' cells, 40‐79 μm3). Compared with synchronous controls (simulated flight conditions), the 55 h postflight recovery period at 1gresulted in a 40% decrease in the A+A' cell population, a 42% increase in the C+D cells, and a 39% increase in the number of PDL fibroblastlike cells near the bone surface. These results are consistent with a postflight osteogenic response in PDL. This recovery response occurred despite physiological stress in the flight animals that resulted in a highly significant (P≤ 0.001) increase in adrenal weight. The data suggest that after spaceflight there is a strong and rapid recovery mechanism for osteoblast differentiation that is not suppressed by physiological stress.— Garetto, L. P.; Gonsalves, M. R.; Morey, E. R.; Durnova, G.; Roberts, W. E. Preosteoblast production 55 hours after a 12.5‐day spaceflight on Cosmos 1887.FASEB J.4: 24‐38; 1990.