Two experiments investigating the effects of short-term spaceflight on cancellous bone turnover were carried out: Physiological Systems Experiment-1 (PSE-1) (a 4-d orbital spaceflight) and PSE-2 (a 10-d flight). Cancellous bone area was not significantly altered by spaceflight in the humerus during either flight. The calculated bone formation rate was unchanged during the 4-d flight but decreased during the 10-d flight. The decrease in calculated bone formation during the longer flight was due to a combination of a statistically significant decrease in mineral apposition rate and a nonsignificant decrease in double label perimeter. The dynamic measurements suggest that spaceflight results in decreases in osteoblast number and activity. Further, the decreases in osteoblast perimeter and osteoid perimeter indicate that a 2-d reloading period was insufficient to restore bone formation to normal. Resorption of the preflight fluorochrome label was not influenced by spaceflight, suggesting that there was no net change in bone resorption. This conclusion is supported by the unremarkable effects of spaceflight on osteoclast number and osteoclast perimeter. Our findings indicate that short-term spaceflight results in depressed osteoblast number and/or activity. Continued decreases in bone formation with normal bone resorption could ultimately lead to cancellous osteopenia and reduced bone strength.