Pure CO was used to euthanatize 18 dogs. During the procedure, physiologic parameters: EEG, ECG, arterial blood pressure, heart rate, respiratory rate, and cortisol values were monitored. Behavioral manifestations were also noted. The EEG modifications indicated a cortical voltage increase followed by rapid cerebral death. Higher heart and respiratory rates during EEG modifications indicated stress, and arterial blood pressure decreased significantly (P = less than 0.05) at the same time. Serum cortisol values were already high before the euthanasia process. Based on these observations, a precise time could not be set for unconsciousness. A gray zone, during which vocalization and agitation occurred for approximately 20 to 25 s, was 3 to 8 s in 10 dogs. These behavioral manifestations could still occur in the conscious phase.