Computer-assisted technology for noninvasive determination of cardiac output (Q̇, Lmin⁻¹) offers potential to re-examine measurement stability with established methods involving CO₂ rebreathing. Reproducibility of Q̇ was examined at rest and during steady-state cycle ergometer exercise (x̄ = 51% of V̇O₂pk; SD = ± 7%) using Defares’ exponential method (MedGraphics™ CPX/D system). Characteristics of the 19 female subjects were: (Mean and SD) age = 34.6 yr and 6.6 and V̇O₂pk= 32.8 mlkg⁻¹min⁻¹ and 9.5. The Q̇ trial consisted of 15 min of seated rest and 15 min of exercise. Values were determined in triplicate at each of minutes, 6, 10, 14. Stability of subjects responses before each rebreathing maneuver was verified by inspection of V̇O₂, Two-way ANOVA’s were performed on the Q̇ data for differences; means (Lmin⁻¹), SD’s, and reliability coefficients were: Rest Q̇-6 min Q̇-10 min Q̇-14 min R Day 1 5.1 ± 0.8 5.0 ± 1.1 4.8 ± 0.9 0.81 Day 2 4.7 ± 0.8 4.6 ± 0.8 4.6 ± 0.8 0.82 Exercise Q̇-6 min Q̇-10 min Q̇-14 min R Day 1 9.14 ± 1.2 9.3 ± 1.4 9.2 ± 1.1 0.93 Day 2 8.6 ± 1.5 8.7 ± 1.4 8.7 ± 1.3 0.97 Day 1 Day 2 R Rest 5.0 ± 0.8 4.7 ± 0.7 0.05 Exercise 9.2 ± 1.2 8.7 ± 1.4 0.84 Although these findings demonstrated within-test and between-test reproducibility of Q̇ to be comparable to that described in the published literature (Kirby TE. J Cardiac Rehabil 5:97-101, 1985) the new semi-automated computerized system (MedGraphics CPX/D) offers a quick, easy, and efficient assessment of cardiac output. / Master of Science