Male Wistar rats prelabeled with tetracycline to mark surfaces of bone and tooth 
formation-mineralization were placed into orbit for 18.5 days aboard the Soviet 
COSMOS-1129 Biosatellite. They were injected with tetracycline for a second and 
third time on the 6th and 27th days, respectively, after recovery of the 
Biosatellite. Spaceflight did not alter the rate of periosteal bone formation in 
the non-weight-bearing ribs and regions of the mandibles, which were covered by 
masticatory muscles. Bone formation-calcification rates were impaired at those 
sites in the jaw that had no contiguous muscle (molar region). The remodeling 
activity on the alveolar bone around the buccal roots of the molar teeth was 
significantly reduced but without creating a negative balance between formative 
and resorptive activities. Total Ca, P, and hydroxyproline concentrations in the 
jaws, incisors, and ribs were normal after spaceflight, but gravity density 
fractionation studies indicated that in the jaws alone, O-G conditions caused a 
delay in the maturation of bone mineral and matrix. A 29-day postflight recovery 
period at earth's gravity was sufficient to fully correct these anomalies. 
Relative to tooth formation, relatively normal circadian and infradian 
biorhythmic periodicities of Ca and P in dentin and enamel were maintained 
during spaceflight. We conclude that most of the non-weight-bearing bones of the 
rat skeleton are at risk to the effects of hypogravity.