Eleven dogs diagnosed with refractory idiopathic epilepsy were treated orally with gabapentin for a minimum of three months at an initial dose of 10 mg/kg every eight hours. They were all experiencing episodes of generalised tonic-clonic seizures and had been treated chronically with a combination of phenobarbital and potassium bromide at doses sufficient to reach acceptable therapeutic serum levels without causing significant side effects. In each dog, the number of seizures per week, the average duration of the seizures and the number of days on which seizures occurred were compared for the three months before and after they were treated with gabapentin. A minimum 50 per cent reduction in the number of seizures per week was interpreted as a positive response to gabapentin, and six of the dogs showed a positive response. After the addition of gabapentin, both the number of seizures per week (P= 0·005) and the number of days with any seizures in a one-week period (P=0·03) were significantly reduced. Mild side effects of ataxia and sedation were observed in five of the dogs, but they were not severe enough to warrant the treatment being discontinued during the trial.