Objective: Evaluation of the accuracy of Masimo Signal Extraction Technology (SET) pulse oximetry in anaesthetized late gestational pregnant sheep.Study Design: Prospective experimental study.Animals: 17 pregnant merino ewes.Methods: Animals included in study were late gestation ewes undergoing general anaesthesia for caesarean delivery or foetal surgery in a medical research laboratory. Masimo Radical-7 pulse oximetry (SpO2) measurements were compared to co-oximetry (SaO2) measurements from arterial blood gas analyses. The failure rate of the pulse oximeter was calculated. Accuracy was assessed by Bland and Altman's (2007) limits of agreement method. The effect of mean arterial blood pressure (MAP), perfusion index (PI) and haemoglobin (Hb) concentration on accuracy were assessed by regression analysis.Results: Forty arterial blood samples paired with SpO2 and blood pressure measurements were obtained. SpO2 ranged from 42 ' 99 % and SaO2 from 43.7 ' 99.9%. MAP ranged from 24 24 ' 82 mmHg, PI from 0.1 ' 1.56 and Hb concentration from 71 - 114 g L-1. Masimo pulse oximetry measurements tended to underestimate oxyhaemoglobin saturation compared to co-oximetry with a bias (mean difference) of -2% and precision (standard deviation of the differences) of 6%. Accuracy appeared to decrease when SpO2 was less than 75%, however numbers were too small for statistical comparisons. Hb concentration and PI had no significant effect on accuracy, whereas MAP was negatively correlated with SpO2 bias.Conclusions: and clinical relevance Masimo SET pulse oximetry can provide reliable and continuous monitoring of oxyhaemoglobin saturation in anaesthetized pregnant sheep during clinically relevant levels of cardiopulmonary dysfunction. Further work is needed to assess pulse oximeter function during extreme hypotension and hypoxaemia.