OBJECTIVE To investigate the effects of intravenous (IV) administration of terbutaline on PaO2, PaCO2, pH, heart rate (HR) and arterial pressures in healthy, laterally recumbent horses breathing ambient air under total intravenous anesthesia (TIVA). METHODS Prospective experimental study. METHODS Eight healthy adult horses were enrolled. Six horses, four mares and two geldings weighing 433-624 kg, completed the study. METHODS Horses were sedated with xylazine (1.0 mg kg-1) IV for placement of arterial and venous catheters. Anesthesia was induced with midazolam (0.1 mg kg-1) and ketamine (2.2 mg kg-1) IV and maintained with an IV infusion of guaifenesin (50 mg mL-1), ketamine (2 mg mL-1) and xylazine (0.5 mg mL-1) at 1.9 ± 0.3 mL kg-1 hour-1. Horses were in left lateral recumbency and breathed air spontaneously. Arterial blood was collected for pH and blood gas analysis during xylazine sedation, 15 minutes after induction of anesthesia, immediately before and 5, 15 and 30 minutes after administration of terbutaline (2 μg kg-1), and when the horse was standing after recovery from anesthesia. HR, systolic (SAP), mean (MAP) and diastolic (DAP) arterial pressures were recorded at 5 minute intervals during anesthesia. Normal data were analyzed with anova and non-normal data were analyzed with a Friedman test with a p < 0.05 considered significant. RESULTS The mean PaO2 decreased from baseline to <60 mmHg (8.0 kPa) during anesthesia (p < 0.0001) and did not improve after administration of terbutaline. After terbutaline administration, HR increased (p = 0.002), and SAP, MAP and DAP decreased (p < 0.001) with the greatest changes occurring immediately after terbutaline administration. CONCLUSIONS Terbutaline (2 μg kg-1) IV did not improve PaO2 and was associated with adverse cardiovascular effects during TIVA in healthy, laterally recumbent horses breathing air.