The plasma immunoreactive adrenocorticotropin and cortisol responses to oral fenfluramine hydrochloride (1.5 mg/kg body wt) or placebo were examined in 11 patients with myotonic dystrophy, 4 controls with facioscapulohumeral dystrophy, a similarly debilitating muscle wasting disease, and 14 normal controls in single-blind studies performed in mid-afternoon. Mean areas under the adrenocorticotropin response versus time curve were significantly greater in myotonics (2573 +/- 429 pmol.min/L) than in facioscapulohumeral dystrophy controls (696 +/- 279 pmol.min/L, P < 0.02) and normals (560 +/- 61 pmol.min/L, P < 0.0001). Corresponding cortisol responses were significantly greater in myotonics (35757 +/- 3949 nmol.min/L) than in normals (21828 +/- 1669 nmol.min/L, P < 0.001), but not significantly greater than those in facioscapulohumeral dystrophy controls (22830 +/- 6140 nmol.min/L, P = 0.055). No stressful side-effects which could affect hormone responses, and no significant changes in blood pressure or heart rate were noted. Fenfluramine activates central serotonergic and/or noradrenergic pathways initiating secretion of corticotropin-releasing hormone and possibly arginine vasopressin. We postulate that these fenfluramine-activated pathways are hyperstimulated in myotonics, leading to adrenocorticotropin and cortisol hypersecretion. This may be a manifestation of a general cell membrane defect in myotonic dystrophy. We found a lack of correlation of age (and severity of disease) with adrenocorticotropin response in myotonics, and therefore, the hyperresponse may serve as a useful marker for the disease before development of other overt signs.