This study examines the one month retention of executive function benefits gained by adolescents with Down syndrome after an 8-week aerobic exercise intervention. Sixteen participants were randomly divided between voluntary (VC) (i.e., self-selected cadence) and assisted (AC) (i.e., 30% faster than self-selected cadence accomplished by a motor) cycling groups, with one participant used as a control (NC). Both cycling groups rode a stationary bicycle, for 30 minutes, three times a week, for eight weeks. At the beginning (i.e., pretest) and end (posttest) of the 8-week session, three executive functions including: set-switching, inhibition, and cognitive planning, were tested. Approximately one month after the posttest, all participants underwent the cognitive testing again. The results showed that for the AC group cognitive planning improved after eight weeks of assisted cycling and these improvements were maintained after one month of no cycling. However, no significant differences were found between the cycling groups for our measure of inhibition. Set-switching appeared to be improved by both types of exercise, rather than only assisted, but the improvements were not maintained during the one month retention period for either group. Thus, our results suggest that Assisted Cycling causes potentially permanent changes in the brain in regards to cognitive planning.