Exercise training has consistently been shown to increase fat utilization during exercise, while conflicting results have been reported on the effects of sex and age on fuel metabolism during exercise. PURPOSE: The primary objective of this investigation was to compare fat and carbohydrate utilization during exercise among males and females varying in age and training status. METHODS: 8 groups of 10 subjects each were formed based upon trained (T)/untrained (U), male (M)/female (F), and young (Y)/older (O): TYM, TYF, UYM, UYF, TOM, TOF, UOM, UOF. All female subjects were experiencing regular menstrual cycles, not using oral contraceptives, and were tested in the mid-follicular phase of their menstrual cycle. The young subjects averaged 21.3 ��1.7 yr and older subjects 40.1 ��1.9 yr. All subjects exercised for 35 minutes on a treadmill at an intensity just below their ventilatory threshold. Substrate utilization was indicated by the respiratory exchange ratio (RER), and a 2x2x2 factorial ANOVA was used to determine whether age, sex, and training status have independent or interacting effects on substrate-utilization variables, and t-tests were used for post-hoc comparisons. RESULTS: The average RER during exercise was lower in trained subjects (0.841��.023) compared to untrained subjects (0.884��.021) (P < .05). There was no difference in the average RER during exercise between the young (0.860��.026) and older (0.865��.034) groups of subjects. The average RER during exercise was lower for untrained females (0.860��.027) than untrained males (0.870��.032) (P < .05), but there was no difference in fat and carbohydrate utilization between trained males (0.843��.023) and trained females (0.838��.022). CONCLUSION: The major finding of this study was that untrained females utilized proportionately more fat during exercise compared to untrained males, but there was no difference in fat and carbohydrate utilization between trained females and trained males. Another finding was that a 20-year difference in age was not sufficient for an age effect to be evident in fat utilization during moderate exercise. / Graduation date: 2013