Haemoglobin mass (Hb mass) determination using CO rebreathing may assist to detect illegal blood doping practices, however variations in Hb mass with periods of intensive training and detraining must be quantified. This study aimed to determine the effect of a 30-day period of detraining on Hb mass in ultra-endurance triathletes. 9 male recreational triathletes (29-44 years) participated in the study. Hb mass was assessed using CO rebreathing 30 days and 10 days before an ultra-endurance triathlon and after ~10, 20 and 30 days of detraining following the race. VO 2max was assessed 10 days before the race and also after the 30-day detraining period, which consisted of an 87% reduction in training hours. After 30-days of detraining there was a 3.1% decrease in mean Hb mass from 86899 to 84094 g, (p=0.03), and a 4.7% decrease in mean VO 2max from 4.830.29 to 4.610.41 L/min as well as a 2.8% increase of body mass from 75.16.4 to 77.16.1 kg and a 28% increase in skinfold total from 43.914.2 to 55.114.0 mm. Individual decreases in Hb mass following detraining would need to be considered if using Hb mass for anti-doping purposes.