Objective:To characterize the effects of continuous positive airway pressure (CPAP) delivered by a commercial human CPAP device on respiratory function in foals with pharmacologically induced respiratory suppression.Design: Prospective randomized, cross-over study comparing CPAP with spontaneous respiration and oxygen insufflation. Setting: University veterinary teaching hospital.Animals: Twelve foals born in consecutive seasons from a university teaching herd.Interventions: Foals were randomized to receive 10 minutes of respiratory support by mask oxygen supplementation or CPAP as a first treatment after induction of respiratory depression by intravenous administration of xylazine and fentanyl. Each foal received the alternate treatment after 10 minutes of breathing ambient air, and the procedure was repeated after 48 hours with treatment order reversed. Measurements and main results:The administration of xylazine and fentanyl by bolus or continuous infusion reliably induced reversible respiratory suppression and recumbency. CPAP was associated with comparable increase in PaO 2 relative to mask oxygen supplementation, but with lower respiratory rate, increased oxygen extraction and increased carbon dioxide elimination.Mild increase in PaCO 2 was observed during CPAP and O 2 supplementation. Expiratory time increased and peak expiratory flow decreased during CPAP. Conclusions:Findings of the study suggest that CPAP might represent a method for improved respiratory support compared to O 2 insufflation due to increased respiratory efficiency. Care must be taken in extrapolation of these findings from foals with pharmacologically induced respiratory compromise to foals with clinical respiratory disease, and further investigation is required to better characterize the cause and impact of marginal hypercapnia observed in these studies. K E Y W O R D Sequine critical care, neonatology, quine respiratory physiology, respiratory insufficiency Abbreviations: ABGS, arterial blood gas sample; CI, confidence interval; CPAP, continuous positive airway pressure; FiO 2 , inspired oxygen pressure; PEF, peak expiratory flow; PIF, peak inspiratory flow; Vt, tidal volume.