A 13-year-old neutered, blue-eyed female Siamese cat with a bodyweight of 4.8 kg was admitted for enucleation of the right eye. An ultrasound guided retrobulbar block with 1 mL of ropivacaine was performed under general anaesthesia. When the tip of the needle was visualised inside the intraconal space, negative aspiration of the syringe before injection and no obvious resistance during injection were confirmed. Instantly, after ropivacaine was administered, the cat became apnoeic, and its heart rate and the blood pressure increased significantly for a short period of time. During surgery, the cat needed cardiovascular support to maintain blood pressure and was under continuous mechanical ventilation. Spontaneous breathing returned 20 min after the end of anaesthesia. Brainstem anaesthesia was suspected, and after recovery, the contralateral eye was examined. A reduced menace response, horizontal nystagmus, mydriasis, and absence of the pupillary light reflex were present. The following day, mydriasis was still present, but the cat was visual and was discharged. The inadvertent intra-arterial injection of ropivacaine was suspected to be the cause of the spread into the brainstem. To the current authors' knowledge, possible brainstem anaesthesia has only been reported in a cat 5 min after a retrobulbar block but never instantly.