We have studied the chemistry, hydroxyapatite crystal size, and maturational changes in bone and dentin from rats exposed to microgravity for 12 days in a Soviet biosatellite (Cosmos 1887). Bone ash was reduced in vertebrae (L5) but not in the non‐weight‐bearing calvaria or mandibles. All tissues had a relatively normal percentage composition of Ca, P, and Mg. Nevertheless, flight rat calvaria and vertebral tissues tended to exhibit lower Ca/P and higher Ca/Mg ratios that any of their weight‐matched control groups, and gradient density analysis (calvaria) indicated a strong shift to the fractions of lower specific gravity that was commensurate with impaired rates of matrix‐mineral maturation. X‐ray diffraction data were confirmatory. Bone hydroxyapatite crystal growth in the mandibles of flight rats was preferentially altered in such a way as to reduce their size (C‐axis dimension). But in the mandibular diastemal region devoid of muscle attachments, flight rat bone and dentin were normal with respect to the Ca, P, Mg, and Zn concentrations and Ca/P and Ca/Mg ratios of age‐matched controls. These observations affirm the concept that while microgravity most adversely affects the maturation of newly formed matrix and mineral moieties in weight‐bearing bone, such effects occur throughout the skeleton.—Simmons, D. J.; Grynpas, M. D.; Rosenberg, G. D. Maturation of bone and dentin matrices in rats flown on the Soviet biosatellite Cosmos 1887.FASEB J.4: 29‐33; 1990.