It has been postulated that the hypoxaemia and hypercapnoea that characterize strenuous equine exercise are partly due to flow limitations imposed by high breathing frequencies (fb), and that gas exchange would be improved if fb could be lowered. To evaluate this possibility, 6 Thoroughbred horses underwent 4 incremental treadmill exercise tests at inclines of 0, 5, 10 and 25%, respectively. In the test, horses were given a warm-up for 2 min, then ran sequentially for 1 min each at 60, 100 and 115% VO2max. Oxygen consumption (VO2), blood gas tensions (PaO2, PaCO2), fb, tidal volume (VT), minute ventilation (Ve), transpulmonary pressure (Ptp), peak inspiratory and expiratory airflows (VI, VE) and work of breathing (Wrm) were determined during the last 15 s of exercise at each intensity. The only effect of fb on PaO2 was seen at 60% VO2max. Also, maximal transpulmonary pressure difference (delta Ptpmax), and peak VI, and VE on a 25% slope were lower than those recorded at the other 3 inclines at 60% VO2max. At 100 and 115% VO2max, the effect of fb was less clear. While fb still differed, the only effects of fb at 100% VO2max were on delta Ptpmax. At 115% VO2max, fb on a 25% incline was lower than that for 0 and 5% slopes. The only other difference noted at this intensity was in VT on 10% slope. However, there was no difference between VTS recorded at inclines of 0, 5 or 25% at 115% VO2max. There was no effect of fb or exercise intensity on Ve at 100 or 115% VO2max. There was no change in PaO2, fb, VT, delta Ptpmax or VI and VE as exercise intensity increased from 60-115% VO2max on slopes of 0, 5 or 10%. However, for exercise on the 25% incline (i.e. with lower fb), each of these parameters increased (or decreased for PaO2) from 60-100%, but not from 100-115% VO2max. Failure of peak airflow and VT to increase when intensity increased was associated with the development of hypoxaemia and hypercapnoea, regardless of slope or fb. It is concluded that while a low fb may have some beneficial effect on gas exchange during submaximal exercise at approximately 60% VO2max, this effect disappears as exercise intensity increases. There appear to be limits to the peak airflows that can be generated by horses during strenuous exercise, and these limits may be reached regardless of fb. Flow limitation per se may play a greater role in the development of exercise-induced hypoxaemia and hypercapnoea in horses than dose fb.