Our purpose was to examine the gas exchange response to exercise in heart transplant (HT) patients and to characterize the O2 uptake kinetics (tau VO2) during successive square-wave on-transients from loadless cycling to moderate exercise. We hypothesized that with a slow heart rate response (and O2 transport limitation) O2 kinetics would be slowed but that with a repeated exercise initiated while the heart rate remained elevated the tau VO2 would be faster. Six male HT patients performed two ramp-function tests to determine peak O2 uptake (1.32 +/- 0.23 l/min) and ventilation threshold (1.02 +/- 0.16 l/min). Patients subsequently completed two repeats of a square-wave forcing function and repeated this on 2 days. Alveolar gas exchange was measured breath by breath. A monoexponential fit of signal-averaged data of the first exercise on-transient (between days) yielded a significantly slower tau VO2 in HT subjects than in healthy men (mean age 47 yr; n = 8) (77 +/- 26 vs. 45 +/- 4 s). With successive exercise (2nd transition) initiated while HR remained elevated the tau VO2 of HT patients was 46 +/- 17 s. The faster O2 kinetics of the second transition suggests that O2 delivery was enhanced and therefore that the tau VO2 may reflect bioenergetic processes controlling the rate of oxidative metabolism.