The abnormal neuronal excitability hypothesized in myotonic dystrophy (MD) might contribute to psychomotor and behavioral disturbances of MD patients. To gain new insights into the pathophysiology of MD, we determined whether the antiarrhythmic drug hydroquinidine would ameliorate slow saccadic eye movements (SEMs), apathy and hypersomnia in MD patients. SEMs were selected as simple modality for psychomotor investigation. The study was conducted in a randomized, placebo-controlled, double-blind, crossover manner. Ten ambulatory patients without contraindications to hydroquinidine administration were enrolled. Hydroquinidine (450 mg/day) or placebo was given orally for 6 weeks with a washout period of 6 weeks between treatments. SEMs were recorded by electrooculography and analyzed by a computer system. Two patients withdrew in the first week of active treatment because of nausea and epigastralgia. Hydroquinidine significantly increased the normalized peak saccadic velocity and shortened the saccadic reaction time compared to placebo. The drug's effects on apathy and hypersomnia are presented in a companion paper.