The dynamic response of oxygen uptake during submaximal cycle exercise appears to be slowed in subjects with type 2 diabetes mellitus compared to non-diabetic controls^1,2. This slowed VO₂ response may be due to slowed responses of cardiac output, limb blood flow and/or oxygen conductance, although human data are lacking. This human study examined the effect of type 2 diabetes mellitus on cardiac output and the dynamic response of VO₂ during three intensities of cycling. Nine middle-aged type 2 diabetic and nine healthy non-diabetic females were recruited for the study. Initially, the ventilatory threshold (VT) and peak VO₂ were determined using a maximal graded cycle test. Then subjects completed a series of constant-load exercise bouts (7 min long) on separate days, during which the dynamic response of VO₂ was assessed three times, and cardiac output once, at each of three intensities (50%VT, 80%VT & 50%[peakVO₂-VT]). Cardiac output was recorded by a closed circuit inert gas rebreathing technique at rest and at 30th & 220th sec of the bout. Dynamic response parameters (e.g., time constant) of the VO₂ response were determined by fitting a monoexponential function to the VO₂ data. Ethical approval was granted by the Trinity College Dublin, Faculty Research Ethics Committee. Results were analyzed using a one-way ANOVA and are shown as mean±SD. The time constant was significantly larger (P<0.05) in the diabetic group compared with the non-diabetic control group at 50% VT (43.9±14.6 s diabetics, 29.3±11.7 s non-diabetic controls) and 80% VT (47±7.8 s diabetics, 35.4±6.3 s non-diabetic controls); but not at 50% [peak VO₂-VT] (47.7±10.1 s diabetics, 47.4±6.5 s heavy controls). Cardiac output responses during exercise (both at 30 and 220 s) were not different between the two groups. The results confirm that type 2 diabetes mellitus slows the dynamic response of VO₂ during light and moderate exercise in females; but that this is probably not related to a slowed cardiac output.