The aim of this work was to evaluate the effects of several inspired oxygen fractions (FiO₂) on the respiratory dynamics in spontaneously breathing dogs submitted to continuous infusion of propofol. Eight adult mongrel dogs were used. Each animal underwent five anesthesias. In each procedure the patient was allowed to breathe a different FiO₂, thereby resulting in five groups, namely: G100 (FiO₂ = 1), G80 (FiO₂ = 0.8), G60 (FiO₂ =0.6), G40 (FiO₂ = 0.4), and G20 (FiO₂ = 0.21). To induce anesthesia, propofol was given until the animals allowed orotracheal intubation, followed by immediate continuous infusion of drug (0.7 mg/kg/min). The initial measurement (M0) of breathing frequency (f), end-tidal carbon dioxide partial pressure (ETCO₂), arterial partial pressure of oxygen (PaO₂) and carbon dioxide (PaCO₂), hemoglobin saturation (SaO₂), base deficit (BD), bicarbonate (HCO₃^-) and pH was recorded thirty minutes after the infusion of propofol has begun. Additional recordings were performed at 15-minute intervals for 60 minutes (M15, M30, M45 and M60). Numeric data were submitted to Analysis of Variance followed by Tukey Test (p<0.05). We recorded significant lower values of SaO₂ at G20, whereas PaO₂ varied according to the changes in oxygen. Regarding PaCO₂ the mean of G100 was greater than G20 at M30. However, spontaneously breathing dogs anesthetized with propofol should not be provided with 100%, 80%, and 21% oxygen due to the significant impairment of respiratory system.