Some cats develop blindness during procedures with mouth gags, which possibly relates to maxillary arterial occlusion by opening the mouth. Our first aim was to use computed tomography (CT) to describe how vascular compression is possible based on morphologic differences between mouth positions. Our second aim was to use nonselective digital subtraction angiography to assess whether opening the mouth induces collateral circulation. Six healthy cats were examined. During CT, the maxillary artery coursed between the angular process of the mandible and the rostrolateral wall of the tympanic bulla. The median distance between these structures was shorter when the mouth was opened (left, 4.3 mm; right, 3.6 mm) vs. closed (left, 6.9 mm; right, 7.1 mm). Additionally, the distance was shorter on the side ipsilateral to the gag (P = 0.03). During nonselective angiography, with the mouth closed, there was strong sequential opacification of the external carotid arteries, maxillary arteries, maxillary retia mirabilia, cerebral arterial circle, and basilar artery. Additionally, there was uniform opacification of the cerebrum and cerebellum. With the mouth opened, opacification of the maxillary arteries (rostral to the angular processes) was reduced in all cats, the cerebral arterial circle and basilar artery had simultaneous opacification in four of six (67%) cats, and the cerebrum had reduced opacification compared to the cerebellum in four of six (67%). In conclusion, the maxillary arteries are situated such that they can be compressed when opening the mouth. Opening the mouth did not consistently induce collateral circulation sufficient to produce comparable cerebral opacification as when the mouth was closed.