A five-month-old female white miniature schnauzer was evaluated for a difficulty in walking, any rapid change in posture or exercise were associated with falling, and bunny-hopping occurred whenever it runs. Physical examination revealed muscle stiffness and hypertrophy, bragnathyism, tongue protruding from the mouth, excessive salivation and increased upper respiratory sounds. Routine hematological was normal and biochemical revealed high levels of creatine kinase (566 U/L, reference value 52-368 U/L). The echocardiography and electrocardiography (EKG) were normal, so, the quinidine treatment (8mg/kg BID) was performed. After the beginning of the treatment the clinical signs became better, and there was an increase of the QT interval by the EKG (from 0.18sec to 0.2sec), but within the reference values. However, the animal had vomit after the treatment with quinidine, therefore this drug was substituted by phenytoin (10mg/kg TID) without any side affects being observed and with a similar clinical affect of the quinidine. The QT intervals remained the same as when the treatment was done with quinidine (0.2 sec.). The animal has been treated for three months with phenytoin, with an improvement in its life condition. The clinical signs observed and the improvement after the treatment which has been applied indicate that it's a hereditary disease of an autosomal recessive condition of the miniature schnauzer breed: congenital myotonic myopathy. The myotonia congenital is characterized by the delay of skeletal muscle relaxation following the cessation of a voluntary activity. This delay in skeletal muscle is not accompanied by any pain or cramping and a mutation in the gene encoding of the chloride channel for skeletal muscles is responsible for the clinical signs which have been seen. It has not been found any report of the phenytoin use in dogs with this disease, and neither any report of this disease in Latin America. Therefore the veterinarians should be alert to this disease with easily visible clinical signs, besides that, the phenytoin could be utilized whenever it is necessary.