To compare the factors of age and gender on aerobic work capacity following bedrest-induced deconditioning, peak oxygen uptake (peak VO2), heart rate (peak HR), and exercise tolerance time were measured in 15 middle-aged men (55 +/- 2 yr) and 17 middle-aged women (55 +/- 1 yr) before and after 10 d of continuous bedrest (BR). The average body weight following BR was unchanged in both men and women. Following BR, peak VO2 decreased from 35.6 +/- 2.0 to 32.6 +/- 1.1 ml . kg-1 . min-1 (-8.4%, p less than 0.05) in the men and from 26.5 +/- 1.4 to 24.7 +/- 1.3 ml . kg-1 . min-1 (-6.8%, p less than 0.05) in the women, while total exercise tolerance time was reduced by 8.1% (p less than 0.05) and 7.3% (p less than 0.05) in the men and women, respectively. The peak HR was elevated by BR from 158 +/- 4 to 165 +/- 4 bpm (+4.4%, p less than 0.05) in the men and from 157 +/- 4 to 159 +/- 4 bpm (+1.3%, NS) in the women. The percent changes in peak VO2, peak HR, and exercise tolerance time measured in the men were not significantly different compared to those of the women. The reduction in peak VO2 in the middle-aged men and women in the present study were comparable to the reductions of 9.3% and 7.8% observed in our earlier studies with 15 young men (21 +/- 1 yr) and 8 young women (28 +/- 2 yr), respectively.(ABSTRACT TRUNCATED AT 250 WORDS)