Ventilation of horse transport vehicles typically relies on vehicle motion to induce air exchange. To assess the adequacy of ventilation in horse transport, air exchange rate was measured in ten locations within a 4-horse trailer, with and without animals present, using tracer gas decay measurements. Three vehicle speeds (13, 48, and 97 km·hr-1) and three window configurations (all windows and roof vents closed, only side windows open, all windows and vents open) were tested. Air exchange rate increased with vehicle speed, and exchange rate at all points within the trailer for the closed configuration was consistently lower than the other two window configurations. The average air exchange rate over all vehicle speeds and ventilation configurations was 0.49 min-1 with animals present and 0.60 min -1 without. The maximum mean exchange rate was 1.42 min-1 at 97 km·hr-1 at the rear left window with all windows and vents open and no animals present; the lowest exchange rate (at the lower position of the rearmost stall divider) was 0.12 min-1 at 13 km·hr-1 with all windows and vents closed. The maximum air exchange rate observed with animals present was 0.84 min-1 with all windows and vents open, and traveling at 97 km·hr-1. Ventilation in the trailer was not adequate when compared to recommendations for stabled horses for any combination of vehicle speed or ventilation configuration.