Sexual dimorphism in body composition and muscle mass is well recognized. The mechanisms responsible for this phenomenon and how a person’s sex affects the response to anabolic and catabolic stimuli are not completely understood. Several studies indicate that there is no difference in the basal rate of muscle protein synthesis or muscle protein breakdown or the anabolic responses to nutritional stimuli and resistance exercise in young and middle-aged adult men and women. Nevertheless, recent evidence suggests that aging affects muscle protein turnover differently in men and women. The basal rate of muscle protein synthesis is greater in old women than in old men; in addition, both old women and old men are resistant to the anabolic effects of exercise and nutrition and old women appear to be more resistant than old men. This suggests that differences in muscle protein turnover between men and women might be most apparent when muscle mass is changing (i.e., during aging vs. young and middle-adulthood when muscle mass is steady) and suggests a possible role of changes in the hormonal milieu with aging, in particular at the onset of menopause in women.