Immunohistochemical methods were used to study the expression of choline acetyltransferase (CAT), 200-kDa neurofilament protein (NF), 28-kDa calbindin (CAB), neuronal NO synthase (nNOS), caspase-3, and Ki-67 in motoneurons in spinal cord segments TIII–TV in male C57/BL6 mice 12 h after completion of 30-day space flights on biosatellite Bion-M1. Controls consisted of mice kept in standard animal-house conditions. The motoneurons of experimental animals showed reactive changes, with increases in motoneuron size, decreases in the numbers in cell subpopulations expressing CAT and NF, increases in the numbers in cell subpopulations containing CAB, the appearance of motoneurons expressing nNOS and caspase-3, and the absence of Ki-67. The absence of proliferating gliocytes and the persistence of the nucleolar component provide evidence of the absence of the necrobiotic changes typical of necrosis. Cytoplasmic vacuolization was seen in motoneurons staining with thionine and immunoreactive for CAB. Degradation of motoneurons into fragments reminiscent of apoptotic bodies was seen in occasional caspase-3-containing cells.