Circulation time (the transit time for a bolus of blood through the circulatory system) is a potential index of cardiac dysfunction in chronic heart failure (HF). In healthy subjects, circulation time falls as cardiac output (Q) rises during exercise, however little is known about this index in HF. In this study we examined the relationship between lung-to-lung circulation time (LLCT) during exercise in ten HF (53 ᠱ4 year, resting ejection fraction = 23 ᠸ%) and control subjects (51 ᠱ8 year). We hypothesized that HF patients would have slower LLCT times during exercise when compared to control subjects. Each subject completed two identical incremental exercise tests during which LLCT was measured in one test and Q measured in the other. Q was measured using the open circuit C2H2 washin technique and circulation time measured using an inert gas technique. In HF patients and control subjects, LLCT decreased and Q increased from rest (HF:LLCT = 53.6 ᠸ.2 s, Q = 4.3 ᠱ.1 l min-1; control: LLCT = 55.3 ᠱ0.9 s, Q = 4.5 ᠰ.5 l min-1) to peak exercise (HF:LLCT = 20.6 ᠳ.9* s, Q = 8.8 ᠲ.5* l min-1; control:LLCT = 14.9 ᠲ.4 s, Q = 16.5 ᠱ.2 l min-1; *P < 0.05 vs control). LLCT was significantly (P < 0.05) slower for the HF group when compared to the control group during submaximal exercise and at peak exercise. However, at a fixed Q the HF subjects had a faster LLCT. We hypothesize that the faster LLCT at a fixed Q for HF patients, may be the result of a more intensive peripheral vasoconstriction of non-active beds and a better redistribution of blood flow.