The purpose of the present study was to measure the time-course and degree of cardiovascular and respiratory 'drift' during constant submaximal exercise in the horse. One Thoroughbred and four Morgan mares were instrumented for simultaneous measurement of respiratory and blood gases which also enabled cardiac output (Q) to be calculated. Data were collected at rest, and at 10, 20 and 30 mins during a constant workload which elicited an initial exercising heart rate (HR) of 150 beats/min, and an approximate 15-fold increase in oxygen consumption (VO2). Significant cardiac and respiratory drift during exercise were observed over time so that ventilation increased from 750 +/- 58 to 910 +/- 49 litres/min (21 per cent increase) from the 10 to 30 min time-point (P < 0.05) and HR went from 154 +/- 4 to 173 +/- 9 beats/min (mean +/- se) over the same time period (P < 0.05). Q also rose from 142 +/- 5 to 177 +/- 17 litres/min (P < 0.05) during the same interval while stroke volume (SV) was maintained. Rectal temperature (TR) and mixed venous lactate (LA) also showed significant increases during exercise while PaO2 and PaCO2 remained constant. The results indicate a significant degree of cardiac and respiratory drift in the horse in response to strenuous submaximal exercise. At the constant exercise work rate chosen, a levelling off, or plateauing of the selected parameters of interest was not observed. Therefore if a true exercising 'steady-state' was achieved, it must have occurred very early in the exercise bout.