This study aimed to characterise both the kinetics within constant heavy-intensity swimming exercise, and to assess the relationships between kinetics and other parameters of aerobic fitness, in well-trained swimmers. On separate days, 21 male swimmers completed: (1) an incremental swimming test to determine their maximal oxygen uptake , first ventilatory threshold (VT), and the velocity associated with and (2) two square-wave transitions from rest to heavy-intensity exercise, to determine their kinetics. All the tests involved breath-by-breath analysis of freestyle swimming using a swimming snorkel. kinetics was modelled with two exponential functions. The mean values for the incremental test were 56.0 ± 6.0 ml min −1  kg −1 , 1.45 ± 0.08 m s −1 ; and 42.1 ± 5.7 ml min −1  kg −1 for , and VT, respectively. For the square-wave transition, the time constant of the primary phase (τ p ) averaged 17.3 ± 5.4 s and the relevant slow component ( A ′ sc ) averaged 4.8 ± 2.9 ml min −1  kg −1 representing 8.9% of the end-exercise (% A ′ sc ). τ p was correlated with ( r  = −0.55, P  = 0.01), but not with either ( r  = 0.05, ns) or VT ( r  = 0.14, ns). The % A ′ sc did not correlate with either ( r  = −0.14, ns) or ( r  = 0.06, ns), but was inversely related with VT ( r  = −0.61, P  < 0.01). This study was the first to describe the kinetics in heavy-intensity swimming using specific swimming exercise and appropriate methods. As has been demonstrated in cycling, faster kinetics allow higher aerobic power outputs to be attained. The slow component seems to be reduced in swimmers with higher ventilatory thresholds.