The optimized carbon monoxide (CO) rebreathing method (oCOR-method) is routinely used to measure total haemoglobin mass (tHbmass). The tHbmass measure is subject to a test-retest typical error of ∼2%, mostly from the precision of carboxyhaemoglobin (HbCO)measurement.We hypothesized that tHbmass would be robust to differences in the bolus of CO administered during the oCOR-method. Twelve participants (ten males and two females; age 27 ± 6 yr, height 177 ± 11 cm and mass 73.9 ± 12.1 kg) completed the oCORmethod on four occasions. Different bolus of CO were administered (LOW: 0.6 ml kg−1; MED1: 1.0 ml kg−1 and HIGH: 1.4 ml kg−1); to determine the reliability of MED1, a second trial was conducted (MED2). tHbmass was found to be significantly less from theHIGH CObolus (776 ± 148 g) when compared to the LOWCO (791 ± 149 g) or MED1 CO (788 ± 149 g) trials. MED2 CO was 785 ± 150 g. The measurement of tHbmass is repeatable to within 0.8%, but a small and notable difference was seen when using a HIGH CO bolus (1.4 to 1.9% less), potentially due to differences in CO uptake kinetics. Previously, an improved precision of the oCOR method was thought to require a higher bolus of CO (i.e. larger %HbCO), as commercial hemoximeters only estimate %HbCO levels to a single decimal place (usually ± 0.1%). With the new hemoximeter used in this study, a bolus of 1.0 ml kg−1 allows adequate precision with acceptable safety.