Multiday endurance sports expose athletes to multiple physical stressors. Little is known about the athletesí physiological responses to these stressors. A detailed understanding of the serological changes that occur during competition may improve the treatment of athletes suffering from illness or injury. This prospective, observational study aimed to characterize serological changes in AR athletes across multiday competition. Athletes underwent venipuncture at the start, midpoint, and end of a 5-day, multidiscipline event. A variety of serological and inflammatory factors was measured and then analyzed to describe their changes over the course of the race. A total of 27 AR athletes (29.6% female, 70.4% male) met inclusion criteria out of 33 recruited initially. The mean age was 37.7 (IQR 32.5, 41). The median race time for athletes was 133 hours (IQR 123, 142). Serum creatinine, sodium, and potassium tended to remain stable as the race progressed. Conversely, serological measures, including hemoglobin, interleukin-6, and C-reactive protein levels, tended to change substantially during the race. Participants demonstrated the ability to maintain homeostasis, despite significant physiological threat. Renal function, electrolyte balance, and hormonal profiles were stable. However, a pro-inflammatory response and decrease in red cell availability were evident by the midpoint of the race.