An abnormal neuronal excitability 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 could ameliorate apathy and hypersomnia besides slow saccadic eye movements in these patients. 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. Apathy was evaluated by means of the apathy evaluation scale (AES) and hypersomnia by a sleep diary. Two patients withdrew in the first week of active treatment because of nausea and epigastralgia. The drug significantly reduced AES scores and daily sleep time compared to placebo. Thus, hydroquinidine can ameliorate apathy and hypersomnia in MD. However, the possibility of proarrhythmia and the high frequency of cardiac disturbances in MD seriously limit the therapeutic perspective. The effects on eye movements are presented in a companion paper.