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Objective To describe the successful management of ascending tonic‐clonic seizure syndrome in a dog after inadvertent intrathecal administration of ionic contrast material. Case Summary A 7‐year‐old, 5.9 kg, male castrated Miniature Pinscher inadvertently received intrathecal ionic contrast material during a myelogram to investigate cervical pain. Ascending tonic‐clonic muscle spasms quickly progressed to generalized seizure activity that was resistant to anticonvulsant medications. The dog developed complete respiratory arrest, which necessitated mechanical ventilatory support for 26 hours. Pneumonia developed and was treated successfully. After resolution of seizure activity and resumption of voluntary respiration, the dog remained tetraparetic for 16 days and was not able to walk on his own for 20 days post contrast injection. Despite a prolonged recovery, the patient survived and recovered normal neurologic function. New or Unique Information Provided Intrathecal administration of ionic contrast material resulting in ascending tonic‐clonic seizure syndrome is rarely reported in the human and veterinary literature. No previous veterinary report has described successful treatment after prolonged respiratory arrest. In previous veterinary reports, patients recovered complete neurologic function within hours to days in contrast to this report in which the patient was tetraparetic for 16 days. This report demonstrates complete recovery from intrathecal ionic contrast administration is possible despite a high dose of contrast and a prolonged recovery.