Muslims abstain from food and drink from dawn to sunset during Ramadam, the ninth month of the Islamic calendar. The purpose of this study was to examine the changes that occurred in body composition and both resting and exercise metabolism during a 4 week Ramadan fast. Subjects consumed an average of 1220 kcal each day during Ramadan and lost a significant 1.92 kg body weight. The subjects also lost 2.8% fat. Fat-free mass did not change. Serum sodium, chloride, and protein increased during the first week of Ramadan and returned to the pre-fasting levels during the last week. This indicates a state of dehydratation existed during the first week of Ramadan. Consistent with this is first week 1.13 kg body weight loss with no change in percent fat. First week Ramadan tests showed a significant decrease in VO2max with a return to the pre-test levels in the last week. The lower resting afternoon VO2 suggests that during Ramadan the body's metabolism slows down during the day in order to conserve its energy stores, however, exercise economy as measured by submaximal VO2 in mk/kg/min is not affected.