ABSTRACT. de Lima M.P.A., Comassetto F., Regalin D., Dallabrida A.L., Ronchi S.J. & Oleskovicz N. [Propofol or midazolam infusion associated with subarachnoid anaesthesia in sheep submitted to bilateral tibial osteotomy.] Infusão contínua de propofol ou midazolam associado à anestesia subaracnóidea em ovinos submetidos a osteotomia bilateral de tíbia. Revista Brasileira de Medicina Veterinária, 38(3):250-256, 2016. Departamento de Medicina Veteriná- ria, Centro de Ciências Agroveterinárias, Universidade do Estado de Santa Catarina, Av. Luís de Camões, 2090, Conta Dinheiro, Lages, SC 88520-000, Brasil. E-mail: firstname.lastname@example.org The sheep stands out for being a great experimental model in the orthopedic area. Thus, the aim of this study was to evaluate the safety and efficacy of the anesthetic maintenance by continuous infusion of propofol or midazolam associated with spinal anesthesia with morphine and ropivacaine in sheep underwent bilateral tibial osteotomy. Twelve healthy sheep, with an average weight of 30.5±2.7 kg were used. The animals were sedated with 0.3 mg.Kg-1 of morphine IM associated with 20 mcg.Kg-1 of detomidine IV. Then they were allocated into two groups: Midazolam group (GMID), which were induced with ketamine 5 mg.Kg-1 and midazolam 0.5 mg.Kg-1 IV, and anesthetic maintenance being performed by continuous infusion of 0 7 mg.Kg-1.h-1 of midazolam; Propofol group (GPRO), which were induced to anesthesia with 4 mg.Kg-1 propofol and maintained with its own infusion at a rate of 0.25 mg.Kg-1.min-1. The animals were intubated and maintained on spontaneous ventilation with 100% oxygen. Spinal anesthesia was performed with 0.5 mg.Kg-1 of 0.75% ropivacaine combined with 0.1 mg.Kg-1 of morphine, diluted with NaCl 0.9% solution to total volume of 1mL/7.5Kg. Significant respiratory depression after anesthesia induction was characterized by significantly increased levels of CO2 and reduced pH in both groups. A significant reduction in heart rate of about 38% after sedation with morphine and detomidine, which remained low until the end of the rating being significantly lower in the GMID. There was no requirement for supplementation during the surgery, and the animals were kept in a shallow anesthesia. We conclude that the use of propofol or midazolam, promoted adequate anesthesia with minimal cardiovascular changes, though occurring with significant respiratory depression.