We evaluated whether ultraendurance swimmers suffer a change of body mass, fat mass, skeletal muscle mass, total body water, and specific gravity of urine during a 12-hr swim in 12 male Caucasian ultraswimmers. Proton nuclear magnetic resonance of urine samples before and after the race was performed to detect alanine, lactate, and 3-hydroxybutyrate. The 12 swimmers achieved an average distance of 29.4 km (SD = 5.1). No statistically significant changes in body mass, fat mass, skeletal muscle mass, and total body water could be determined (p greater than 0.05), but urine gravity decreased significantly (p less than 0.001). Two participants showed increased signals of ketone bodies after the race. In these samples, the hydroxybutyrate-creatinine ratio increased more than tenfold. (Contains 5 figures.)