Forty-four college women trained on a bicycle ergometer 2 times weekly for 10 weeks. Each session consisted of one continual “all-out” ride at a cadence of 60 rpm, beginning at a work load of 360 kgm/min. The load was increased by 180 kgm/min every 2 min until the subject could not continue. Changes which occurred after training included higher maximal values for pulmonary ventilation, oxygen uptake, oxygen pulse, heart rate, and total work output, and a lower maximal ventilation equivalent. Immediately after the termination of training the subjects were assigned at random to 2 experimental groups which were then randomly assigned to 5-or 10-week detraining periods. The detraining periods produced increases in resting heart rate and maximal ventilation equivalent and decreases in total work and maximal values for pulmonary ventilation, oxygen uptake, and oxygen pulse. Losses in maximal values for oxygen uptake, oxygen pulse, and ventilation equivalent were greater for 10 than for 5 weeks of detraining. Improvements which occurred in maximal values for heart rate, pulmonary ventilation, and total work following training were retained to some degree after 10 weeks of detraining.