This study was carried out to examine changes in pulmonary vascular resistance (PVR) induced by moderate and strenuous exercise; the objective being to understand why pulmonary artery blood pressure of exercising horses increases progressively as work intensity increases. Pulmonary arterial and wedge pressures (referenced at the point of the left shoulder) were determined simultaneously with cardiac output in 2 groups of healthy, sound, exercise-trained horses. Horses in Group 1 (n = 8) were studied at rest and during exercise performed at 8 and 13 m/s; the latter workload eliciting maximal heart rate (mean ± s.e. 212 ± 3 beats/min). Horses in Group 2 (n = 7 Thoroughbreds) were studied at rest and during galloping at 14.5 m/s on 5% uphill grade, a workload which elicited maximal heart rate (217 ± 3 beats/min) and could not be sustained for >90 s. Pulmonary vascular resistance was calculated by dividing pulmonary perfusion pressure gradient (i.e. mean pulmonary arterial pressure minus mean pulmonary wedge pressure) with cardiac output Pulmonary arterial and wedge pressures, pulmonary perfusion pressure gradient and cardiac output increased significantly (P<.05) with exercise in both groups. There were no differences in PVR between the 2 groups of horses at rest In Group 1 horses PVR decreased significantly (P<.05) with exercise at 8 m/s, but further pulmonary vasodilat0ion did not occur as workload increased to 13 m/s. During exercise at 14.5 m/s on 5% grade, PVR of Group 2 horses also decreased significantly and was not different from values for 8 or 13 m/s in Group 1 horses. It is concluded that PVR reaches its nadir during moderate exercise, presumably due to upper limit of recruitment and distension having been reached. Therefore, in accordance with Ohm's law (P α Q × PVR), in going to higher workloads pulmonary blood flow (Q̇) remained the sole determinant of the rise in pulmonary arterial blood pressure (P). Our data also indicate that pulmonary artery wedge pressure is another variable that is important in determining the absolute value of pulmonary arterial blood pressure.