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The effects of thiopental, ketamine, diazepam, xylazine and nitrous oxide, and combinations of thiopental-nitrous oxide and ketamine-nitrous oxide on electroencephalographic (EEG) spike activity and convulsive behaviors in atropinized cats at surgical depth of enflurane anesthesia were assessed quantitatively for 60 minutes during spontaneous ventilation. Mean inspired enflurane concentrations (MIEC) were reduced 16% to 29% by pretreatment with thiopental, ketamine, diazepam, and xylazine, and were reduced 19% by 66% nitrous oxide. The MIEC of cats anesthetized with thiopental-nitrous oxide-enflurane and ketamine-nitrous oxide-enflurane were 35% to 38% lower than that with nitrous oxide-enflurane. Pretreatment with thiopental, ketamine, diazepam, and xylazine did not reduce the EEG spike frequency during anesthesia but did markedly reduce the spike amplitude. The addition of 66% nitrous oxide did not alter the spike frequency during anesthesia but tended to reduce the spike amplitude. Combinations of thiopental-nitrous oxide and ketamine-nitrous oxide almost abolished the spike activity. The addition of 66% nitrous oxide prevented convulsive responses elicited by photic and auditory stimulation during enflurane anesthesia. Treatment with thiopental, ketamine, diazepam and xylazine, and combinations of thiopental-nitrous oxide and ketamine-nitrous oxide, completely prevented convulsive responses during enflurane anesthesia.