We are reporting on a 46-year-old man who has suffered of muscle cramps for 4 years, occurring immediately after jogging and playing tennis and lasting for 7-8 hours. Repeated neurological, orthopedic, internal medical and endocrinological examinations showed no pathological findings. Physiotherapy, supplementation of fluids and electrolytes had no effect, nor did medication therapy with muscle relaxants. During spiroergometry without medication, there was an overproportional increase of heart rate and respiratory rate with delayed pCO2 increase after exercise with otherwise normal blood gas levels. This reaction was considerably reduced during spiroergometry under beta-blockade (metoprolol 100 mg); at the same time, the muscle cramps could no longer be induced. Both excessive respiratory regulation and direct hyperadrenergic stimulation should be discussed as the primary cause of the muscle cramps. According to recent findings, b-blockers with intrinsic sympathocomimetic activity should be avoided in therapy.