Oxygen is used for medical treatment and general anesthesia. However, high concentrations of oxygen can have toxic effects on cells. In veterinary medicine, 100% oxygen is usually used during general anesthesia and it can be toxic to animals. However, there is little concern about its harmful effects in humans. The objective of this study was to demonstrate that using a high con- centration of oxygen increases the partial pressure of oxygen in arterial blood (PaO2) more so than a lower concentration, by comparing PaO2 at three different oxygen concentrations (100%, 60%, and 40%) in six dogs under general anesthesia for 3 hours. The mean PaO2 and standard error values at the 100%, 60%, and 40% oxygen concentrations were 535.8 ± 24.01, 374 ± 17.19, and 239 ± 8.78 mmHg, respectively (p<0.05). These results show that 100% and 60% oxygen concentrations could increase oxidative stress. Further studies are needed to examine the oxygen concentration that causes toxicity.